Clearly, sometimes it's inevitable, as you can't be at two or more places at the same time. So as long as you do it as soon as possible, with a good explanation (getting another interview that you cannot reschedule is fine, I did it a few times), you should be fine. Sometimes, however, a school simply won't be able to offer you a different spot, in which case you have to choose wisely. As far as them sounding enthusiastic, don't count on it, since it is a lot of hassle for them, but it really doesn't matter - usually you are making harder the lives of administrators and not the committee members.
As far as cancelling interviews because you got into a school you know you would rather go to (I did this a few times as well), I would argue you are not only allowed, but definitely should do this, as you are being only fair to both the school and other applicants who might or might not otherwise get to interview there (and are really interested in the school).
As far as doing it, a short email is usually fine, but some schools will ask you to send in something in writing in order to withdraw.
Good luck and congratulations on being in a situation to worry about this 😀.
