Can a university rescind a confirmed offer?

I got into my firm, but I'm considering calling up the university to see if I can swap course. If I call up and they don't have places or won't let me swap, can they take away my original offer? I'd really like to call and try but I don't want to put my current place in jeopardy.

No they shouldn't take it away if you ask to swap so don't worry.

Thank you - I only want to change from single to joint honours so fingers crossed!