The B6 situation is precisely why CO moved in the first place. Massport had to beg CO to move - I believe they offered them a sweetheart deal to do so. I think UA may have some extra space in C, but my guess is that there simply isn't enough room in C for CO to move back to. Were I a CO flier out of BOS, I wouldn't particularly want them to move either, as Terminal A is easily the nicest terminal at the airport and Terminal C might very well be the worst.