This is a very interesting post. Thanks to all who have contributed so far. My robovac recently developed a similar fault (a few weeks out of warranty). So I am looking at doing this repair.
I managed to get the gear off mine fairly easily. I used a pair of crocodile nose pliers. I managed to get the thin end between the gear and the motor housing and with some wiggling and pushing with leverage managed to get the gear off in just a few minutes. No noticeable damage to the gear either.
Was thinking of replacing the wheel motors as well. Are these the same ones?