Somewhat similar situation happened to me in ATL 6-7 years ago travelling to SJU. I was booked in F (B fare and upgrade at TOB about 1.5 weeks before flight) and I already had a BP with seat assignment. About 40 minutes before departure I heard my name being paged to the counter. Not expecting this and not knowing for what reason they could be paging me to the desk I timidly go up to the counter expecting to be told to call home for an emergency (this is before I carried a cell phone). The agent asked if I would mind giving up my F seat for an $800 voucher and fly in Y. I was so happy that I was not being told a member of my family had died I said sure but how about if I just fly on the next flight in F and the agent said "no problem". After he gave me the voucher I asked why he had picked me to ask and he said he had a family traveling together (paid F) and I was the only single passenger close to where the other family members were seated. That satsified me that it had nothing to do with me being on an upgraded fare but who knows.