Go take a look at how the original cast is doing. Let's just say with the exception of Mr. Ford, they are all quite...err... comfortable.
Bringing them back now would be akin to casting Marlon Brando in the new Superman movie. Outside of wink wink nudge nudge it's not a good idea.
Not necessarily. I could see Mark Hamill playing a role similar to what Alec Guinness did in the original Star Wars.