Switch sim cards or go to the AT&T store?

I have 2 iPhones (say A and B) on family plan (both are eligible for upgrade). I am planning to replace one of them (say old iPhone A that is damaged) by new 4g. The problem is that i also need to switch numbers (new iPhone 4g should get number from old iPhone B and old iPhone B should get number from old iPhone A). Should I just switch sim cards between 2 old iPhones before activating new 4g? Or should I activate 4g first with old iPhone A number and then go to the AT&T store asking them to switch the numbers? Thanks.