Did you do the same day change at the airport, or did you change your flight to a different date? You can also just call AC RES with your locator number and they can tell you.

It does sound like the agent stuck you in tango in either case, if it was the same day, then it can be fixed. If you changed to another date and you did ''buy'' a tango fare (agents fault or not) you cannot do anything about it. Always confirm your class!!