University of Edinburgh

How hard is it to change subjects in Scottish unis?

I'm an American prospective student, and I'm looking at applying to Edinburgh , St. Andrews, and Glasgow, but I'm facing quite abit of problems knowing that it is much harder to switch subjects once you are accepted into a course. This bothers me since I've always wanted to attend the aforementioned universities and I certainly don't think I am set on a single
subject, so can anyone explain how difficult it is? I feel that they are much less rigorous compared to their English peers, but say, is it possible to switch from a language/humanities subject (french, art history) to a science/engineering subject (mechanical eng, physics, etc.,)? If so, what chances do I stand? I have good qualifications to get accepted either into an engineering/science course or a language (especially for french, arabic), does that help much? Thanks!