If you mean once you've started the course then you can change..
http://www.bia.homeoffice.gov.uk/stu...s-to-a-course/
But you still need to receive permission for the BIA for your new course. It depends on your circumstances whether you are allowed to start the course before you actually receive permission. Though even if you have started the new course they could refuse you permission and therefore your visa might become invalid if you cannot go back to your old course.
