You don't say which flight had a time change. As per regulation EC261/2004 (see relevant threads), the operating airline of the flight which had a time change needs to reaccommodate you on the best available itinerary. If BA changed their flight time it is them, if it is AA which changed their flight time then it is their problem and you should contact them. They may refuse at first but do mention the regulation, and they will likely become more cooperative. BAEC will likely only offer you a routing with redemption seats available. which is unlikely to be in your interest.