A co-worker did his at work today. He had to call AT&T before activating the phone. The first lady he got was clueless as to what to do, just kept telling him to switch out the sim card with his old one and wouldn't understand they are different sim cards. So he called back and got a different lady who understood and knew what to do. She walked him through it, giving her the ID's of the phone (they are on the iPhone box so have it handy) and the phone number he wanted on it. Then he had to keep the iPhone plugged into iTunes while they do their stuff of switching out the number. However, he didn't leave it plugged in long enough and it caused the sim card to corrupt. He had to go to the AT&T store and they gave him a new sim and did the swap there.
Overall it was actually pretty easy and quick for him, besides the issue of him unplugging it to early before it finished but others have reported no issue with the upgrade swap and doing it yourself.