If you are traveling with a non-medallion companion you have to call to have their seat moved to a premium seat. If you do it yourself online it will not hold. I have done this many, many times and have never had to give an explanation or been refused. I just tell them that I would like to move my travel companion's seat next to mine and they are happy to help.

The good thing for the travel companion is that if you get upgraded they get to keep the premium seat without you. I always book 2 aisle seats if they are available to keep my companion out of the middle seat alone in the event I get my upgrade!