Contrary to my initial post, I have been informed that there are no credit checks when upgrading your phone. What they do is verify your payment history on your account and make their decision based on that. It never goes to a full on credit check whenever your upgrading your phone. I apologize for my previous post and have removed it and corrected myself here.
I was wondering about this today and it completely answered my question. I'm thinking of getting a new phone because my current one still works, but it's getting old and the charge drops really rapidly when I talk on the phone. It can go from full charged to half charge in just over half an hour.