*sigh*
I apologise for coming across bitchy, but what exactly is your motivation for changing? What frustrates me is a lot of people who try and get into a course via a "back door" approach, which means that the rest of us who applied and withstood the competition feel annoyed.
To answer your question: Yes, it is possible, someone on my course changed from Biology to Economics half way through the term. I remember that her reason amused me greatly (in the sort of sense that makes you want to hurt something afterwards) but she did it nonetheless.
The university doesn't like it, but I think it may be easier to change when you are actually here, and not before. It's up to you, but I have to warn you that this year they are giving fewer Economics offers as they gave out too many last year and 200 people turned up instead of the anticipated 130