There's a few things you can do before blindly paying:
#1 - look at your original reservation confirmation email. the fare rules there would be clearly spelled out. especially if you purchased the ticket in the UK, it's the law. It would be in the fine print at the bottom.
#2 - you could try to end-run the agent (Assuming you've bought from the agent), and try to change the ticket on the AA.com website or through AA.com customer service, but you may end up in the same situation because your original booking agent will probably have an imprint of your credit card, and will know your fare change and penalize you accordingly.
#3. If you find the after all the changes and other usuary charges, the ticket becomes really expensive (ie 2K to return to the UK), you could just abandon the ticket and buy another 1-way ticket home. There should be no penalty for a no-show. Though you may want to check. The cheapest way to get home to the UK right now would be to actually buy the ticket from a local agent - as they can try to get you on a thomson or other holiday charter flight back to the UK.
#4. in searching alternative flights back as a one-way ticket purchase, also consider that you don't have to leave out of the same airport. there are 5 good airports here that fly internationally. and getting around the country here to get a cheaper international flight is not uncommon. so aside from the local agent, try
Matrix - ITA Software ITA software search engine recently purchased by google. You can put in 'SDQ' as the airport, and then pick all the airports that are 150km close by as your starting point. Similarly, you can set the destination airports being not just London, but anything that's up to 300 km away. Do you really care if you land in berlin and can take a 20$ ryanair flight back to somethingshire? if not, then consider this option too...