Congrats on the upgrade & new status PaceLaw! I'm a *G who travels US from time to time, and really like their A321 product. Here's what you should know:

1) Yes, you can change them via your 'my reservations' on US.com--it won't affect your upgrade.

2) Go-go is unfortunately not free in first, so you'd have to buy a hour, flight, or day pass.

3) You won't have any lounge access with your F boarding passes, but you can buy a day pass (2--one for you, one for her) or if you have an AMEX Plat card you can get in for free with up to 2 guests (or immediate family).

4) On the 321, 4D/F are certainly not bad seats, but most prefer row 2 or 3 if it's available. 1 is a bulkhead which means your legroom will be slightly tighter and you'll can't store your bags in front of you during t/o & landing--not a huge deal again but it makes all the other rows preferable. I wouldn't give up sitting next to each other for rows 2 or 3, but if 2 together are available in the middle of the cabin, I'd definitely switch.

5) I'd say be glad you got this upgrade, and continue to fly US--their ug system is more generous in my experience than this board gives way to, so I'd expect you to have continued luck.

Happy flying!

CSurfer