I think what is going to happen is that Colbert will create a new "character" for this show. He'll shift his act at least somewhat, as that's what the gig demands, but he'll still have an on air personae that is not necessarily the "real" Colbert. He acts a certain way to accomplish what he needs to for the show on which he is working. Nothing wrong with that. I think he'll be an adequate replacement to Letterman, but I thought Ferguson or Louis CK would have been, too. I don't think Louis CK could have done it very long term, though. I think he needs more change than a show like that would offer him. Would have had a great couple of years, though.
But it is an interesting thought - have four hosts, have them rotate weeks each month. Could be interesting. Although, what got Letterman his long-term audience is how consistently he did things. He would start doing some bit, and you'd think, "What the heck is this?" And then, three weeks later, you'd be saying, "Hey, where is that bit? He's been doing it every day..." Part of Letterman's charm - do something until it is funny. Takes guts.