Magic Key holders can cancel a park reservation or remove a guest through 11:59 p.m. Pacific on the day of the visit.
The instruction appears in the official Magic Key how-to guide. Disneyland directs passholders to My Theme Park Reservations when they need to cancel an existing reservation.
The deadline carries a policy caveat
The guide adds that a different cutoff may apply if one is specified by the no-show policy in effect at the time. Passholders should check the current terms before relying on the 11:59 p.m. deadline.
The wording specifically covers canceling a reservation or removing a guest. It does not establish that every other reservation change follows the same deadline.