I think this occurred at an outstation. I suspect that there weren't redcoats and the GA might have been an airport contract agent who doesn't handle DL flights often. It might have been necessary to call the DM line from the gate. This is why Isaid that I suspect that the GA didn't realize that the OP was awaiting a FC seat assignment, not in need of a coach seat.
I wonder whether the flight left with empty FC seats or whether nonrevs were put into the FC seats available at the gate. Maybe someone here can look that up since the flight was yesterday.
I was also tempted to say something about why the OP didn't challenge the seat assignment, but the thread title is making me wonder whether it's an extremely young OP (minor means under 21?), but if someone does 200 segments per year (and is a DM, which implies either not cheap tickets or an AmEx card, so it's unlikely to be a student who travels a lot) commuting, this would seem unlikely (but possible).
OTOH, if the word minor in the thread title means trivial or insignificant, I would disagree. The GA stole the OP's upgrade.