Thanks for the great responses. I really appreciate it. I thought that switching GRAs is not a big deal but I contacted the program admin and apparently they don't encourage students to take on a project that they are prepared to abandon once you have better things to do. Students often switch because they no longer are interested in the research and that is a legitimate reason.
I do have a lot to think about. His research is very interesting to me but I would be lying if I definitely want to pursue this for my Ph.D. May be because I'm a little bit alienated to his field. I only took one course in undergraduate related to it.
Also, I just heard back from my top priority professor and he's saying if I decided to come, he would like to talk to me. He just doesn't have any GRA opening until next year. Does that rule out what TakeruK pointed out: professors tend to give priority to incoming students? If that professor is serious about it, I think I have a very good chance next year.