Your best bet would be to try to make the city change at the airport...with an airport agent...not even mentioning it over the phone. The problem, in your exact situation now is...the phone agent may have documented your reservation (PNR) in the computer saying that the passenger was advised changes permitted only to the date/time for $75...but no change to other cities.

NEVER "ask" about changes like this (e.g. things that are really not allowed) over the phone!!! They can document your record, and then you have a less likely chance doing it at the airport....because the airport agent will say "oh it says right here you were advised you cannot do that."

Now its possible that reservations did not document your record. You might call and casually ask if there are any special notes in your reservation about making a change....etc. Have to very carefully ask this question...and add to this line above...but it can work to see if they have documented your record.

In an ideal situation, you would want to just go to the airport, try to pick out an agent that you think looks like they are really not too much into the rules, and ask them to change the city for you.....siting some circumstance that just came up at the last minute. Granted, above posters are correct....this is not the policy of the airline....and you only have like maybe a 50 50 or less chance of it working...but stranger things have happened.