I have seen at least 3-4 threads asking the same question so I thought I would make this one to try and consolidate all of them. I asked the manager from my AT&T store and he confirmed that it IS possible to use someone else's upgrade and put it on your number. The best way to do this is to go to an AT&T store instead of an Apple store as Apple cannot switch the numbers around. If you get your iPhone from Apple, you can take it to an AT&T store to get the numbers switch but they will charge you a $18 activation fee.
Hope this clears everything up
Please keep this near the top to avoid similar threads from being created
Hope this clears everything up
Please keep this near the top to avoid similar threads from being created