I always “traded” my upgrade for a service man/woman in uniform. It was especially fun in Raleigh NC where Marines just out of boot transiting and heading home (my God they looked young !!!).
The second best part about doing this was that the gate agents (US Air and United were my main carriers) usually found another upgrade and I still got to ride up front.
I can only speak for those two airlines, but they could not have been nicer to our guys.
Southwest out of San Diego had a Marine employed at boarding when my son graduated. He told the pilot and they actually called his name and everyone applauded. I laughed, he blushed, but he was very impressed. The Marines do not wear their cammies in public.