You can:
pay the change fee
or
buy a new ticket and cancel the old one
or
travel on the flight as stipulated on the ticket you purchased wrongly

agents cannot override the system and change tickets against fare rules because you ‘made an honest mistake’, what’s the point of fare rules otherwise?

it doesn’t matter why you want the ticket changed, all that matters is you want them changed and according to the fare rules which you have agreed to abide to upon purchasing your tickets, any change before departure will incur a set change fee and any fare difference. It’s all embedded in Amadeus (global distribution system), when a change is initiated, a payment dialogue will prompt the agent to collect payment. Computer says no so what can you do?