I think unless the feedback on changes is harshly negative (which I doubt, as even here in grogland it's broadly positive with some concerns), we'll be looking at much larger changes than 3.5, overall, particularly in that I suspect more classes and subclasses will be tweaked/changed than 3.5, and they'll be changed more than most classes were in 3.5.
So I think the situation will be a bit different. It'll be more like the 1E/2E situation. In 2E you could pretty much just run 1E classes/races (like those in Oriental Adventures), with close to no work, but you did notice, and sometimes it did create issues, and they were only compatible with themselves. But because D&D uses this exception-based model that kind of doesn't matter unless you try do something like apply a 2E Kit to a 1E class.