What happens if I have to cancel?

The traveller is refunded in full, always, whatever the tour's cancellation policy says. That policy governs what a traveller gets back when they cancel, not what happens when you do.

Cancelling is sometimes unavoidable and we would rather you did it early than went ahead with a tour you cannot run. Do it as far in advance as you can: it is somebody's trip, and the further out you cancel the more chance they have of finding another guide.