Short answer: You most likely can re-book your tour.
Longer answer: It is impossible for manta ray tour operators to guarantee manta rays sightings every night of the year and it is possible that you’ll be unlucky and join a manta ray night dive/snorkel without seeing a single manta ray.
If this happens it is standard practice for the tour operators to either re-book you free of charge or to give a large discount for a tour one of the following nights. However, you should check these conditions yourself before booking.
This means that it is wise to plan your manta ray night dive early during your stay so that, if necessary, you can reschedule your tour without running into too many conflicts with the rest of your itinerary!