A guest wants to change the date or group size. What do I do?
It is your call. If the new date is open and the group still fits the boat, it is usually easier to move the trip than to lose it.
Date moves are generally worth accommodating — a rescheduled trip is still a trip. Group size is different, because it can change the price on per-person trips and it can run into your legal capacity.
Capacity is a hard limit. No guest request makes it safe to exceed it.