Converting GOP Senators

Just prior to the election, there was an NY Times article about Lincoln Chafee, the uber-moderate Senator from RI (R), with some speculation as to whether he would switch parties. Now there's a dust-up over Arlen Specter and his pro-choice stance.

Does anyone know here about the possibility that either of these two (or anyone else-- perhaps in the House as well, though I imagine that's likely) would switch parties? How does this happen? When would it happen? Would the Democrats have to offer Committee seats? Could they?

Any information will be greatly appreciated.