In between your wilder thoughts about this you actually have the right answer.

When there is an equipment swap to another aircraft type, the point being with a different seat plan, AA cancels the original flight and replaces it with a 9601 type of flight number. Basically it avoids seating clashes, as well as dealing with the potential for downgrades and also possible "oversell".

Nothing to do with LHR obviously, given the range of airport pairs that 9601 served in your screenshot.