I don't think they let you swap until the first few weeks of term are over, then people tend to move around a little bit. We had people move into my building from Pollock and people drop out etc, so there is some moving space, but I seem to remember being told there was a policy for no room changes until a few weeks after you'd moved in. I'm not sure what they'll do now if you decide not to accept the accommodation, I believe if you do not accept it you are no longer guarenteed the offer of a place so you'll have to do with whats left, so therefore might end up with a twin room again anyway. If you're really not happy then contact them though, obviously.
At least you've not been allocated Chancellors, this year the first years were paying ridculous amounts to share bedrooms in CC with bunk beds, which is crazy for the amount of money they pay for the rooms. The same happened the year above me too.