Allowing clients to reschedule appointments but not cancel appointment

[TW91]

My clients prepay for all of their appointments or class bookings.

I want to set up a 48hrs rescheduling window - allowing them to be able to reschedule upto 48hrs before their appointment.

However I have a no refund - no cancellations policy. So I don't want them to be able to cancel their appointment. 

How/where do I set this up?