BA can't force AA to open up award availability on the EWR-MIA flight. So the agent was correct. There needs to be award availability on the flight you want to be moved to, even with the cancellation. It can be the small downfall of booking partner award flights. You can try call up again and ask the agent to speak with AA, but once again it's not guaranteed.

What is the date of your flight? Presumably quite far in the future?