Hey there, Dena! Thanks so much for stopping by planDisney with your ticket reservation question.
Oh boy! I'm so sorry to hear that plans for tomorrow aren't working out and that the majority of your party won't be able to show up for your park day. It sounds like you have a plan and to me, it sounds like your plan works. You should be able to go in and cancel all the reservations you need for the tickets of the party members who can't attend while still keeping the one reservation for the person going. Once those reservations are canceled, you can go back in and schedule new reservations for a new date for the other tickets.
You can adjust
theme park ticket reservations as often as needed during the validity of the ticket for one-day tickets either in the
Disneyland app or on the Disneyland website with the Disney account the tickets are linked to. If the tickets are multiday tickets you can change reservations as well, but once you have used one day all other days must be used within 13 days of that first day of use. Just make note that if the tickets are one-day tickets, you will only be able to make reservations for days that match the same tier of the ticket or a lower-tiered day.
Dena, I hope this information is helpful and you can get your reservations rebooked and sorted out. Please come back and visit us again soon if you have any other questions! See you real soon!
Carrie