1. I would nonetheless have your colleague added to the UG list. Perhaps you will both be upgraded. One never knows.
2. Presuming that he is not, simply let the FA at the boarding door know that you have swapped. That is both a courtesy and guards against that one FA somewhere who tells you to sit in your assigned seat, in which case that is what you do.
The chances of a problem are somewhat beneath miniscule and people do this every day.