Switching research topics to something less interesting?
If you want to do research on X, you should go to a place where you can do that! Going to school A, working on Y, and hoping that you might be able to switch advisors does not sound like a good idea. How much the professor at school A wants you should not really matter, you have to do what is right for you; he will find someone else. Moreover, I would not already think about what to do if I lose interest in X. This is hypothetical anyway, and in this case, there might be some topic Z that is more interesting than Y etc.