When at top of UG list:

I always ask if F has boarded full, if yes then I board to my C+ seat.

If I find upon entry to the aircraft that there are empty seats in F I return to the GA and challenge her false statement.

Some who I met before will advise me to board and promise me to come on board to upgrade me in case of no-show. They are usually good with this promise.

If there are empties and rookie GA brings manifest on board I will challenge them. I always ask for their name when told to board.

VDB:

Offer to volunteer but do not surrender your original BP. State firmly that you want your original seat if your volunteering is not needed.