Originally Posted by Finkface

Yes, you can definitely upgrade companion fare tickets using miles or GGU's. There must be U space available for both people (search for 2 seats on your flight choosing 'MVP Gold Guest' option on the left where it says 'upgrade option' and make sure the U isn't greyed out with a line through it. And you must have the instruments (miles or GGU's) for both people as both people must be upgraded at the same time.

If using GGU's, you can do this online. Look on your reservation and at the top, it will say MVP Gold Guest Upgrade. Click on that and it will allow you to choose the segment(s) you want to upgrade and you just input the certificate numbers. Not sure about upgrading with miles, whether you have to call in or if you can do it online. I think you have to call in, IIRC.