The part is bigger than the actor so recasting is possible for these characters as has been said James Bond has been recast many times same with Dr Who that is even written into the lore of the character. Other characters are the actors though like with Jack Sparrow, that character was created by the actor not the writing.
To make a great film you need three things - the script, the script and the script -Alfred Hitchcock

Although Ruffalo is good in the role, he definitely doesn't conjure up the image of the scrawny nerdy scientist from the comics that is associated with Bruce Banner does he?

I thought it was a shame they'd already used Sam Rockwell for Justin Hammer as he's basically Edward Nortons twin and would have made a good banner replacement.
Ruffalo is good, he just doesn't look like Banner from the comics even a little.

Thanks to Batmeh v Supermeh Yawn of Justice, the "S" now stands for Sidekick

"It's me, I'm here, get over it"
Exactly!

I liked Cheadle's intro in IM2..
"Rhodey? this isn't like you"
"It's me, I'm here, get over it"

Thanks to Batmeh v Supermeh Yawn of Justice, the "S" now stands for Sidekick

James Bond moved on without Sean Connery. Iron Man can survive the loss of RDJ no matter how much he has become synonymous with the role.

That depends who ur recasting I think, they couldn't just recast Iron Man and go on like nothing happened, RDJ has become too iconic in the role.