Well, before going all Terminator on them a few things to consider, the same fare class being available doesn't mean there is no fare difference as surcharges and fare basis could differ. So the question arises does the same fare basis remain valid for your new dates for instance (blackout dates and such). If for some reason there's a difference in fare basis then perhaps the cost is the same but rules differ, or the rules have changed for the same fare basis, in which case I'm not sure which would prevail.

I'd have a look on ExpertFlyer what the current rules for your fare are. Lastly you say you were repeatedly told, how many times did you call? I'd honestly try call back again and request for a team leader to review the fare rules and pricing.

Ultimately if you live in a country where these things are easy (UK might be one) just file a chargeback for that portion of the fee.

Does it charge the fee if you try change it online (if you are able to)