I had to reschedule a few shops during the last couple years and had no problems. Of course I emailed the scheduler the day the shop was due and explained my problem (car broke down, heavy rain/flooding that day, turnpike was closed, etc.) Generally, I requested the shop be rescheduled for the next
day. I don't think they would be as understanding if they found out you were not planning to do the shop on the original scheduled date.