I once flew LGA-MCI and looked at the TV screen for information about upgrades.
The plane returned MCI-LGA (or possibly MCI-JFK) later that day, using the same flight number.
There were some open F seats, as there tends to be on this elite-light route.
I believe the cost of upgrading LGA-MCI was significantly more than LGA-MCI-LGA/JFK! They presumably looked at the net displacement to determine how much to charge.