I think you have legitimate cause to argue for paying what ever bag fees were in place for tickets bought on your original buy date and flown on your date of departure. Not sure what US was like, but most carriers had a phase in, where tickets bought on/before XX date but flown before YY date weren't assessed a fee, and tickets bought on/before XX date but flown after YY date were assessed the fee. Find out what the policy was at the time. If it is different then what would apply if you bought the ticket today, bring your original itinerary when you check in and hope to get a sympathetic agent.