I was on a flight about 20 years ago which was almost full. There was 1 seat left in F and a bunch of hopeful upgraders at the counter (old days - different procedures for upgrades at the gate - you had to wait outside or else you lost your place on the list). My client was in coach, I was in First. It was sweltering. I got up from my seat and walked back to the counter (yes, you could do that 20 years ago) and said "I want to pay the difference for my client who is sweltering in 21C, please list him DSR." The GA finished the transaction in about 30 seconds and gave me the new boarding pass to hand to my client. Two of the people, who were waiting for the upgrade to clear, said "Nice move" with a genuine smile.