So i'm running into a very similar problem - booked my parents on award tickets to LHR (daytime flight via ORD) - for the end of May.

A couple of weeks ago AA cancels the first flight of the day to ORD and hence the only remaining way to get to LHR during the day is to go direct on AA*/BA flight - managed to get an agent to make that change as the only alternatives were >6 hours from the original ticket.

Just got a call from them today, saying the only way they can accommodate me is to charge me an extra $500 in "taxes" or fly with horrible layovers and overnight flights with hotels at "my own expense", or if I refused those two options they would simply refund my miles.

After arguing with them for quite some time and eventually speaking to a condescending supervisor was told my choices were refund miles, pay the "taxes", "ok we'll call them a fuel charge" to fly on BA or take a horrible connection.

I refused all of the offered options, and asked to speak to someone who could waive the additional charges - they said they'd call me back at some point.

Anyone got any bright ideas about my rights here?