The easiest way to do an upgrade swap is to upgrade the lines that are eligible for the upgrade, then swap SIM cards (either there or when you get home).
I recently got the White iPhone 4, my mom's iPhone 3GS died and rather than pay the out-of-warrenty price at Apple to get it exchanged, she decided to use my dad's upgrade to get herself the iPhone 4.
We went to Best Buy because they were the only ones that still had 16GB iPhone 4's in stock (everyone now only has 8gb), They upgraded my dad's line but put my mom's SIM card in the phone, and as soon as we got home my mom and I exchanged phones (I put my SIM card in the White iPhone and she put her SIM card in the Black iPhone, we restored from back-ups with no issue).