Actually this exact issue has happened for me and I faction changed from a dwarf (Not DI) to Troll (then I racechanged to UD as its this character).
I managed to get a few quests done and now im sitting on a quest log full with quests that cant be completed.
And I have had 3 (!!!) game masters checking it with only the last one getting remotely close to helping with the issue.
To note: This ONLY HAPPENS TO CHARACTERS WHO WERE FACTION CHANGED TWICE, ie from Horde to Alliance and then back to Alli. I have faction changed numerous times and only in 8.1 did my quest progression blow up.
Mind, my stint with Customer Support has been mindboggling. Here’s the ticket log for the sake of some comedy at the start of the year:
Since I faction changed today almost all of my faction and war story quests have reset.
However, almost all of them cant be completed. The storyline quests in each zone are bugged as Im in a phase where there is no NPC I can interact with. The war story quests are the same with chapters I cant complete due to the hand-in NPCs being phased out.
All of these quests Ive already completed once on this character prior to my move to the Horde. Id like to ask all these quests to be completed (and I will gladly forfeit their wards) or have my faction change reverted so I can play on Horde where I am not stuck at various stages of the war campaign and zone questlines.
Response:
Hi there Kruule,
I’m GM XXXX, I am here to look into your quest log after a faction change,
Sorry I didn’t get chance to talk with you online this morning, I hope you had a great week.
When you faction change this does have a major impact on your quests, as warned in the FAQ’s for the service. Basically every quest chain and zone which is faction specific completely resets. To unlock everything again you need to start for the beginning and redo the starting and zone quests. This is sadly a WAI situation.
The problem is that even if we undo the faction change and move you back to Horde, the horde quests have already reset as well, you will need to redo these again.
Sorry for the problems this causes you. have a most excellent week.
My response:
Hi there,
Thanks for the response!
I forgot to add that this issue persis on Kruule (now Zuldarak, as I faction change) where the same problem arose as you mentioned.
However, I have done faction changes in the past and in those cases the number of quests that reset were marginal: It was only the dungeon quests at the end of each zone and I had to readd my outposts on the enemy faction’s continent.
Now I have an entire sleugh of quests and questchains, including the war campaign. During my last several faction changed playthroughs I had completed every storyline in each zone as well as additional quests that arent part of the main questchains.
This issue was not present up until now and I would like it resolved and not rely on what you mentioned regarding the issues with faction changing, mostly because:
A. This issue was NOT present when I faction changed previously. Regardless of what the FAQ stated I didnt have to redo complete zones or the war campaign when I had already done those particular quests in 8.0 or even 8.1.
B. The reset affects the main storyline, which means I CANNOT finish the campaign of the expansion and continue the story whenever it gets new chapters added
C. For all intents and purposes phasing issues should NOT break the game to this extent as to make dozens of quests impossible to complete.
At the moment I have an abundant number of quests in my questlog, ready for turning in, only to have the NPC not present or uninteractable at their location, as well as numerous quests I cant start (visible on the map as yellow exclamation marks) because the questgiver is phased. Kindly look into this and please dont resolve the ticket until I can get my quest log (and the expansion’s quest and story content) as clear as it was in my prior faction changed times.
And here’s where this issue was addressed in the most lackluster way possible:
Greetings XXXXXX,
I’m XXXXX from the Greater Azerothian Material Entities for the Mythical And Sentient Tranquility and Equality of all Reality department, otherwise known as G.A.M.E.M.A.S.T.E.R.s.
I can understand that you are frustrating, that you need to do the quest lines again. However the answer my colleague has given is correct. With the release of Battle for Azeroth the impact on Faction Changes is much more servere and as we told you already the quests have been reset. This is not a bug where we can help. This is a Working as Intended Scenario.
As i can see you have already started the War Campaign on Horde side. Here is a link, where in the comments you can see the Questline for your current quest “Azerite denied”: https://www.wowhead.com/quest=53852/azerite-denied
You can see the War Campaign Quests on the Map as Orange Quest Markers.
Unfortunately this is all we can help you with. you will need to continue the War Campaign and Quests on your own.
I will keep my fingers crossed and wish you a relaxing weekend and have a happy new year.
Best Regards
Game Master XXXXXX
Customer Support
Blizzard Entertainment
GM Joke of the Week: How many Dwarfs do you need to screw in a Light Bulb?
100! One who holds the Light Bulb and 99 which drink until the room spins 
Naturally my response to that was less than polite because at this stage it became clear these people don’t read the web tickets at all.
Hi xxxxxx!
Thank you for the reply,however I aam not entirely thankful for the fact you didnt read the contents of my ticket. Parroting the same thing your colleague stated and disregarding the state of my character and what I had written isnt exactly something I expected.
“I can understand that you are frustrating, that you need to do the quest lines again.” I’m glad for your understanding, but as I stated:
However, I have done faction changes in the past and in those cases the number of quests that reset were marginal: It was only the dungeon quests at the end of each zone and I had to readd my outposts on the enemy faction’s continent.
Now I have an entire sleugh of quests and questchains, including the war campaign. During my last several faction changed playthroughs I had completed every storyline in each zone as well as additional quests that arent part of the main questchains.
“However the answer my colleague has given is correct. With the release of Battle for Azeroth the impact on Faction Changes is much more servere and as we told you already the quests have been reset.”
This is also factually wrong as I stated:
“However, I have done faction changes in the past and in those cases the number of quests that reset were marginal: It was only the dungeon quests at the end of each zone and I had to readd my outposts on the enemy faction’s continent.”
What you said WAS NOT PRESENT AT THE LAUNCH OF THE EXPANSION. This issue occured after 8.1 started!
“This is not a bug where we can help. This is a Working as Intended Scenario.”
Not only is this ill informed, its also clear you didn’t even check my character.
I have a full questlog now (barring 2 slots for the weekly or call to arms quests) with quests that are ready to be turning in BUT THEY CAN’T BE FINISHED BECAUSE I FACTION CHANGED. How can you willingly state that this is a working as intended scenario when this wasn’t a scenario up until now AND NONE OF THEM CAN BE COMPLETED?
“As i can see you have already started the War Campaign on Horde side. Here is a link, where in the comments you can see the Questline for your current quest “Azerite denied”: https://www.wowhead.com/quest=53852/azerite-denied”
Feel free to take control of my character and head to the questline’s location. I will be interested in seeing you complete it when Rexxar is phased out.
Other than that I have 2 other War Campaign quests active from other stages which also can’t be completed BECAUSE I HAVE DONE THEM ALREADY AND THEY ARE PHASED!
“You can see the War Campaign Quests on the Map as Orange Quest Markers.”
As I also said:
At the moment I have an abundant number of quests in my questlog, ready for turning in, only to have the NPC not present or uninteractable at their location, as well as numerous quests I cant start (visible on the map as yellow exclamation marks) because the questgiver is phased.
“Unfortunately this is all we can help you with. you will need to continue the War Campaign and Quests on your own.”
I would love to continue them IF THEY COULD BE CONTINUED. READ THE TICKET PLEASE! INVESTIGATE! MY CHARACTER (Named Hierarch) IS AVAILABLE to your heart’s content to verify my claims.
Here’s a link for the sake of showing I’m not the only one who has this problem:
https://www.reddit.com/r/wow/comments/a8vz51/allied_race_faction_change_in_81/
If you want I can upload an onslaught of screenshots too of the different questing areas and how the NPCs related to the quests aren’t there or cant be interacted with. As one user stated: “Had something similar. Reported the bug and they fixed it asap.” If this was true, I am hoping for the same treatment, not for a wish-wash, uninformed statement based on false evidence.
Best regards,
XXXXXX
After New Years I actually got a gamemaster who at least managed to address issue amongst the 30 I have at the moment:
Greetings XXXXX,
I’m Gamemaster XXXXX and would like to thank you for contacting us.
I just checked your issue thoroughly once more and hope, you had a nice start into the new, despite the answers of my colleagues. I would like apologise, that you were not happy with those.
During the investigation I could see, that you indeed were phased out from the wat campaign of 8.0 and due to that I now completed this part of the campaign for you.
The campaign continues in 8.1 with “Our War Continues” and I placed you right in front of Sylvanas, so you can start this. 
I hope, this answer helps you and wish you a lot of fun.
After which I responded, being very thankful, but ultimately still in need of assistance because I have the other 30+ quests which cant even be completed.
Hi XXXXXXX,
First of all, happy new year and thank you very much with completing some of the War Campaign quests that couldnt be completed!
However, the issue persists. As I mentioned, my questlog right now is full with quests that cant be completed because of phasing issues. All the ones I have end in an area where the questgiver isnt available or cant be interacted with.
Additionally, It Belongs in My Mech! is the next quest I have and its also not completable as Im in a phase where the quest objective (Azerite Mucks) arent there. These questchains remain incomplete until I get a helping hand with either auto-completing them (Ive done them before on this Warlock as a Horde char, FCed to Alliance, then they reset after I faction changed back to Horde) or to have a fix for the phasing issues.
Thank you in advance for any assistance.
TL;DR: So a fair warning! If you decide to faction change, do your best not to regret it! Because if you want to switch back on the same character oh boy, are you in for a wild ride of incompetency!