You can change degrees fairly easily but, depending on the requirements for your new degree, you may have to fulfill a number of additional requirements. For example, if you are going to a large public university and are in their engineering program studying for an electrical engineering degree, you can switch over to computer science with very little difficulty. However, if you are in the liberal arts college studying philosophy and want to switch to computer science in the engineering college, you will often have certain minimum requirements to be accepted into the college (much like if you had applied from outside).
Overall, though, US universities and colleges are very flexible compared to other countries. Probably why they are so popular with foreign students. :)