Having seen the inside of the new core books, I'd say it's already a tough sell just for them.
Despite all the good things they've done with them, I'm sorely disappointed that the MM isn't laid out with one monster per page. The background lines in the PHB are still there. Silly, little quirks like those that should have been a no-brainer are still in the new revised books. Does WotC think these things are helping gamers?
Don't get me wrong, I'll still buy them, but part of me is groaning over the stupid little things they SHOULD have made time to fix.
Hardbacks are just too expensive to repurchase without a good reason. The 3.5 revision has too many changes to be able to write-in the changes manually. So, I'm buying the new books.