Following up::
I called the QME Doctor's office and said :I have a commitment out of town until _date_, could I please reschedule before or afterwards- Or will I need to do this through my WC adjustor?"
they said, "Don't worry, how does _date_ work for you? Good. We'll call your adjustor and let them know about the change."

All the same, I think I'll call my adjustor myself (maybe after business hours cos we aren't exactly friends) and leave a message that I re-scheduled and if there is any problem let me know.