Unfortunate situation, and especially surprising with a company like Avis. Then again, agents typically get commission for upgrades, so given human nature, you're always going to encounter a bad apple here and there.
Two things to mention:
1. Sometimes rental companies will record the interation at the rental counter between the agent and the customer. It's possible that was the case here, so you might want to press them on this front since if it went down as you say, there would be a record of it.
2. You could try emailing
Larry De Shon (
[email protected]), President of AvisBudget and/or
Joe Ferraro (
[email protected]), President - Americas.