excerpt from my $ILL email receipt. They leave themselves wiggle room to reschedule but it doesnt seem right for them to change the time instantly after booking.
Individual Lightning Lane
Non-refundable, non-transferable, and revocable, and may not be redeemed for cash in whole or in part.
Attractions and experiences (collectively, "experiences") and arrival windows are limited, and subject to availability and change, downtime, delay or closure without notice or liability, and entrance to an experience is not guaranteed. Experiences may vary by date, are subject to change, and may not be available at all on the date of visit or at the time you make a purchase. Availability of experiences and arrival windows can change throughout the day. Disney reserves the right to modify, re-schedule, re-assign or cancel any experience selection in the event of a change in operating hours, experience downtime or closure, or park closure. In the event of cancellation by Disney of an individually purchased experience selection, Disney’s responsibility and liability, if any, shall be limited to a refund of the amount paid for the cancelled experience.
Each purchased experience selection allows one (1) guest to enter the experience using the Lightning Lane entrance (or other designated entrance) once on the date of redemption during regular park hours and during the selected arrival window only. The Guest may not enter before or after the arrival window, and the experience selection expires at the end of the arrival window. Guests may not modify experiences or arrival windows once purchased, and may not reschedule an expired experience selection.