Currently Schedule Assistant is designed so that when you use it on an already booked job, it resets the Arrival window so you would have to repick it if you wanted to keep the job booked at the same time.
If you pulled the arrival window into Schedule Assistant when using on an already booked appointment, it would allow tenants to use ACP to book a job for an available timeslot and Schedule assistant to find the best technician for that appointment specifically.