Is it going to be like any upgrade in game where the base items move to its higher version?
Or items magic bonus will get upgraded as well?
Because I don’t see the usefulness if it is simple base item upgrade. Who will use upgraded Saigon over IK for example.
I’m not 100% sure about the actual physical upgrade bit but really want to see it in. A nice short ladder with upgradable sigons does appeal to me. I’m pretty sure the bloke said that partial and set bonuses would be upgraded though.
Base item gets upgraded. Stats remain the same.
Its useful for i.e. belts, to get more space for potions, higher def.
Noone would prefer Sigons over IK, except in some situations. The intention to upgrade set items is NOT to make new “endgame” items.
Just to make them more viable for early/mid game.
Someone who has a Saigon and doesn’t have an IK?
We don’t all trivialise the game by trading for BiS. Some players enjoy the challenge of using only what they find.
h ttps://en.m.wikipedia.org/wiki/Saigon_(disambiguation)
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Why 95% of you guys pronounce Sigons Set wrong?
I don’t think they said they would limit the upgrade to just a base upgrade. That’s not going to change anything at all for most (all?) sets.
They just say the want to expand the current system for uniques onto sets. So its likely it will stay the same.
And this should be enough basically. There are many mid-game runewords. Plus upgraded sets in early/mid game is enough.
As mentioned, they dont want to revamp the whole itemization. They just want to make the old sets a little more viable. Depens on the needed stuff for upgrading then.
What I took away from it is there will be two things happening to sets:
There will be a few more set bonusses added to sets, not completely sure but way it was talked about they were mainly talking when have two or three set pieces, not sure if also full set bonusses will be improved. Well impact on this will be mostly dependent on those bonusses, may be underwhelming or overwhelming depending on the added bonusses;
Adding the ability to upgrade the base item up to elite. Well for defence based items this can mainly be fun for belts and add some to even a lot of defence, Orphan’s call Guillaume face will still have no life leech though unless added as a set bonus. 
Weapons though that can make quite a big difference, it is also the damage base of the item that improves that improves and some sets and set items have decent damage bonusses. Hwanins Bill for example might not become OP as such unless it gets an insane part set bonus and still lose easily out to runewords for polearms, but its base damage wilgo up by a lot when upgraded and same will go for most low level set weapons, Szasabi anyone, so it may have way more impact as we suspect even if not OP and mainly very nice for early hell and before high runewords as an extra option.