I would definitely call again. The agent may simply have been wrong about the $500 to change.
One thing though, you said that there were $800+ in taxes on the ticket. Are you sure about that? BA like to group part of the fare into the 'taxes+carrier-imposed surcharge' part. Check whether part of the $800+ you mention are 'fuel surcharge (YQ)' or similar. These are not taxes: it's part of the fare that BA have split off and therefore won't be refunding to you on a non-refundable fare.