I don't think recasting any of the characters would be the end of the world, but I have different thoughts on different characters.
I want Eustace to be recast. This is nothing against Will Poulter's performance in VDT, but he is simply too old. Narnia is about children going to a magical land, not 20-somethings going to a magical land. In LWW the actors ages were good, but they were a bit too old for me in PC and VDT. Because the kids were older in PC, the tone was changed to more "action fantasy" than "fairy tale" (I'm sure there was more to this than the actors ages, but that was probably part of it) and in some ways felt less like a Narnia story than LWW. VDT had Georgie Henley and Skandar Keynes act like children, but this led to awkward performances.
I like Liam Neeson as Aslan, but I wouldn't mind seeing a new actor's take on him. An actor who could bring more ferocity to the role wouldn't be a bad thing.
I wouldn't want the Pevensies to be recast. Their roles in HHB and LB are so driven by nostalgia for the earlier books that it wouldn't make sense to have unfamiliar faces in the roles.
I would like to see Ben Barnes return as an older Caspian just because I really liked him in the role, especially in VDT, but I wouldn't be heart-broken if they recast. Obviously he will be older, so I could see the film-makers thinking it would make more sense to cast someone who is actually Caspian's age than using extensive makeup for Ben Barnes.
I don't think it makes sense to bring Peter Dinklage back as Trumpkin. He would be more expensive now after
Game of Thrones and I don't think his small role in SC is enough to justify bringing him back. Not to mention the single Trumpkin scene ("What's that? Who's been killed?"
) in SC has Trumpkin acting very differently from how Dinklage portrayed him. He's basically comic relief in SC and it isn't the dry humour of movie Trumpkin.
I think James McAvoy depends on whether or not they get Georgie Henley back. If she comes back, I wouldn't want anyone else playing Tumnus. Otherwise, I'd be fine with a recast.