It really comes down to how they want to frame the revamp.
Is Azeroth corrupted or does she remain pure?
If corrupt, do we cleanse her and if so when?
Is she literally removed from the planet as a Dark Titan, or does she remain inside?
Does the revamp emphasise the elemental and "earthy" part of Azeroth, is it more avoid the world-soul and how it affects everything. Void and light effects? The Emerald Dream seeping through? It's hard to tell.
A revamp could begin as a corrupted version of Azeroth which we purge to make it look pretty. If so, what WE think of as a revamp might take place at the end of the expansion.
A revamp could be pure and pretty right away, either at the start or the end of the expansion.
What we do know is that thousands of assets have been remade over the past five years, suitable for zones like Ashenvale, Dun Morough and Elwynn Forest as they appear today. Intact buildings, beautiful effects, etc.
This leads me to believe that we start off the revamp journey in 10.3 or 11.0 with an either corrupt or neutral Azeroth. The revamp might happen right away, but it also makes sense if we first have to purify Azeroth and restore her. That would make for a very satisfying revamp storyline. Azeroth is "reborn" or "finally purged" after all these years.