There is an old interview where they stated that raids would scale to appropriate levels like they do now… Like Molten Core would now be 20, LK would be 25, etc etc.
(not sure the exact levels).
As others have mentioned, Chromie time is a different scenario, but once you exit, I can imagine if Molten Core is lvl 20 (for example - not 100% sure on the level), then all of the vanilla zones would only go up to lvl 20. I don’t have any concrete links talking about it, but I doubt they’d make MC lvl 20, but make Stranglethorn go up to lvl 50.
Granted, this is an old interview, but I haven’t seen or heard anything different since.