Facts
1) You do NOT have to go to the store
2) You do NOT have to swap sim cards
It's easy, all you do is order the phone online, using the upgrade you have available no matter who's line it's on.
Then upon receiving the phone you take it out of the box, charge it, call your carrier and say you want to do an upgrade swap and they do it all via computer.
There is no need to make this ten times more complicated than this simple solution.
If you use my solution you will spend about 10 minutes of your time on the phone for free.
No gas to burn, no store to drive to, no wasted BS.