FWIW: So like everyone else I was scared to transfer my upgrade to another line, use it, then switch the phone back. Well it worked, and I didn't even lose unlimited on the line I switched the upgrade to! I've waited a few days, checking online just to make sure and it still shows both lines as Unlimited Data!! The guy at the corp. store was just as surprised as I was when it didn't ask to switch the other line to a tiered plan, and at that point I wasn't asking questions. I was told that switching an upgrade then using it and switching the phones back would keep your unlimited though!