I don't think they would bother putting "Revised" on the front cover of the book unless at least SOMETHING significant was changed.
Since the information we have now says changing some of the classes will be rebalanced and revisions and additions to spell lists, if that is true alone it makes it somewhat incompatable with previous products without sort of a "Well, THIS NPC bard doesn't have the new power from the revised book because...he's special" or without changing things.
If I remember correctly, they said all sorts of the same things about the revised Star Wars Core Rulebook, that it was completely compatable with previous products, which is only a half-truth.
Yes, all of the NPCs still worked in the new system, they would have the same class names, but different powers and you'd have to give them all the powers they deserved from the new edition.
I don't think that the changes will be QUITE as comprehensive as the Star Wars ones, but will still make some changes to classes, probably only 2 or 3 of them, but those classes will have the same problem as happened in Star Wars.
I will probably get them just because I want to see if the new classes are better or worse than the old ones, and I will likely make due with the fact that I have to convert NPC stats from old adventures and such. If they fix the couple holes in the rules, I'd even like these books. Still, I'm not going to like paying for the books if what is in them is just a recap of all of the errata and questions available online, reprinting a bunch of feats and prestige classes from the splat books, and then a couple changes to the classes that could have been made into a web enhancement.
Majoru Oakheart