Rescheduling frees your lesson time and returns your credits in full straight away, at no cost. You then have 14 days to book a new lesson with any teacher who takes that class, the teacher you first booked included. If you have not booked one within 14 days, half the credits are kept.
If you reschedule 48 hours or more before your lesson, nothing is held back. If you reschedule less than 48 hours before it, half the credits stay at risk until your replacement lesson has actually taken place. Take that lesson and nothing is kept. Cancel it, miss it, or never book one, and half the credits are kept, exactly as if you had cancelled late. This can only cost you half of one lesson, never more, and it follows your booking if you reschedule again.
If your teacher ever needs to cancel, you always get your credits back in full.