It appears you cannot change the difficulty from Normal to Heroic while in ICC anymore. Therefore you cannot use a raid lockout from another character to skip to Lich King for the Invincible reins.
Thanks Blizzard, I love spending 30 minutes to clear out old raids!
That’s weird, I used the skip mechanic yesterday (after the pre-patch hit) and it worked as before. The only difference was that I had to physically leave the instance (step outside) before I was able to change the difficulty. That was it. So unless they changed it since yesterday it’s still very much useable.
Well I learnt about this skip one month after I got Invincible on my 89 try. 
I did the usual method - create a group on my normal locked character, log out to my farming character request invite to that group, accept group on locked character, fly to ICC on farming character enter the instance, accept the 10/12 locked instance wait until it passes me leader then usually you right click your character set it to heroic and then kick out the other character. I’m now unable to do that and I did what you said before, leave and re-join but it enters a 25 heroic version with no bosses killed.
Can you share your method, perhaps I’ve gotten a step wrong?
Pretty sure something similar applies to SOO and Garrosh now. Used to be that you could have a HC lockout with just the big guy left - which alts could run for a chance at the tusks of mannoroth. AFAIK that is no longer possible.
You can no longer change the difficulty while INSIDE the instance, but when you step outside you can change it. Once outside, right click on your character portrait like before and change it. Step back inside and you’ll get the message with 11/12 (or 10/12) bosses and you can accept. That worked for me.
And here’s me, farming the mount since late cata, with hundreds of LK kills across multiple characters, without even knowing about the skip…
sigh
Could you explain the steps 1 by 1 please I’m having a hard time understanding your new method, thanks.
/pat
Seriously, don’t feel sorry, I was farming the damned horse (pun intended) for a long time before my friend, who started playing in Cata (or even in MoP), told me about it… I felt so silly.
I’ve been farming this damn horse since early MoP and I only learned about the skip early BFA. It’s a nice time save but I just want this damn horse to drop I farm it on 10 characters every week.
Sure.
Done.
As I said, the only thing that changed is that you can no longer change the difficulty while INSIDE the raid (the option is greyed out). Once you step outside the option is clickable again. Happy farming!
Okay I’ll try it after this full ICC run I made this thread halfway through. If it still works, then that’s awesome but I’m still very confused as to why they made this slight change.
I made it my weekly routine (everytime i had an an active account), i didn’t even bother to check for updated tips or skips…serves me right i guess 
I don’t know why they changed it either, but the reason I immediately tried that was that you had to step outside to change the difficulty during the earlier expansions as well. The option to change it while inside is something that they ‘recently’ implemented (from Legion onwards I think?). I was used to having to step outside, so that’s why I immediately tried that. I hope it works out, it would be weird if they changed this after so many years.
At least you got a lot of transmogs that way! 
I’m sure I own nearly every transmog piece for all my characters I run it on, I wish I still had the rarity add-on installed to see how many total attempts I’ve had must be in the low 400s.
Cheers. Takes bit more time now, I usually wait for leader change at Lich King, but beats doing whole raid.
Okay so I did it your method, when I set the raid to heroic after being given the leader outside, I walk in and it asks me if I want to accept I do but I notice the raid difficulty is still on normal 25 and the portal is the same as normal. It appears your method might have been removed too. I’ve tried it 4 times and cannot get it to work.
I’ll try it again with another character as soon as I’ve levelled it to 50, as I said it worked like a charm yesterday (managed to kill Arthas five times).