Under the circumstances the fact that you now need two segments to complete the trip you booked won't be a problem. The AA agent was within his/her bounds to offer you the LHR connection. Since you can't use that routing, just call AA's ATW desk again, explain the situation, and ask for the two segments that *do* work for you. Should be a piece of cake. Of course, you may find an agent or two who doesn't understand the situation, but in the end it will be okay. When an OWE airline has some operational change - be it late equipment or a schedule change - they generally do what it takes to get you where you want to be regardless of OWE rules.