If they stick to the backwards compatibility claim* then I expect to be able to use any 5e adventure and
almost any 5e supplement with minimal effort on my part - and I expect that effort to be detailed in a revision document available
for download from one of their sites (a PDF from D&D Beyond is fine; having to actually visit DDB is not, for reasons I've detailed in other threads).
So if I can run "Curse of Strahd", but have to use a revised stat block for Strahd then that's fine by me, provided they provide the replacement.
By
almost any supplement I mean anything except "Volo's Guide to Monsters" and "Mordenkainen's Tome of Foes", as those two have been explicitly deprecated. And I have no problem completely switching out the core rulebooks.
* Speaking for myself, I wouldn't have a problem if they end up allowing scope creep to move this closer to a 5.5e edition than has previously been indicated, and thus abandoning backwards compatibility... provided they state that up-front. Just don't claim it and fail to deliver!