It might have been easy(ish) for Marvel to get away with switching Ed Norton for Mark Ruffalo when it came to Avengers casting, but a large part of the audience's acceptance of that probably came from the perceived knowledge that Bruce Banner is a lot like James Bond in that he can look extremely different from movie to movie (less so in that Bond doesn't turn into a huge, green rage monster or rock purple shorts quite so well as Dr. Banner). It wouldn't be quite as easy to replace, say, Robert Downey Jr. as Tony Stark without a massive fan backlash akin to every Batman casting announcement ever made. Marvel are all about the continuity in their movies and TV output at the moment, so they likely wouldn't be interested in doing a straight recasting with this much invested in a universe in which so much of their audience is readily invested in. Instead they'd probably go the Battle for the Cowl route (to use an inappropriate, somewhat ham-fisted reference) in attempting to pass the mantle on to another character. So we might yet see another actor in the Iron Man suit if RDJ retires in the next few years, but it could be a previous supporting character like James Rhodes (War Machine, currently played by Don Cheadle) or even - and this would be fantastic - Pepper Potts (Gwyneth Paltrow). That way Marvel get to keep their billion-dollar franchise characters going and fans don't have to moan about said character being ruined by atrocious casting. Everybody wins!
