Accepting and then declining PhD offer, I recently got accepted into PhD programs at University A,B,C, and D.

I interviewed with Professor W at University A, Professor X at University B, Professor Y at University C, and Professor Z at University D.

I chose University B.

But now I am having doubts about B  and wish to go to University A to work with Professor W.  How do I tell Professor X that I want to change my mind and work with Professor W?

I also had additional conversations with Y and Z at C and D; they were pretty helpful and gave good guidance and tips about going to A and not going to B, so I feel even stronger about my decision to attend A over B.  

University C and University D are on the east coast, while A and B are on the west coast.

How should I handle this?  

Will I be burning bridges with X, Y and Z at B, C and D? 

I wish to do my post-doc someday at University D or maybe C so it's important to not piss off X and Y, who both know B very well since the area of research is quite small.

W also knows X Y and Z
As you as you are professional and upfront, this shouldn't cause any hard feelings. Go with the one you like the most, after all, it is your research and time that you will be spending. You can always change your mind and email professor at A with a brief message explaining that you prefer to go to a different school. Keep in mind that you will be just starting your PhD and who knows if you will have the same opinion about going for a post doc or maybe get lucky and find a faculty job! Opportunities can rise out of no where. 

One more question, why did the other professors suggested that you go to A and not B or even their university? Do they know the professor at A? Or because of the university's reputation or lab?