Hello Amber,
While I am sorry to hear some of your family is missing a haunting of a good time at Mickey's Not-So-Scary Halloween Party, I am happy other family members will enjoy this BOO-tastic event. I, too, had family members who were unable to attend and through My Disney Experience account was able reassign the tickets to others.
First, be sure those you are reassigning to have a My Disney Experience account and send them a
family/friend request so you are able to link reservations and tickets. Second, make your way to the
Walt Disney World website and scroll over the My Disney Experience tab. Third, select My Reservation and Tickets. If you have already assigned the tickets, here you will see your Mickey's Not-So-Scary Halloween Party tickets and who they are assigned to. Fourth, select the person you are wishing to change, keep in mind and Adult ticket needs to go to an Adult and a Child goes to a Child. Fifth, you will see an option to reassign, select this ticket and follow the prompts to reassign your ticket to another family member.
If I can be of any further pixie dusted assistance feel free to ask and may your upcoming stay at Walt Disney World be filled with 999 happy haunts and many magical and memorable moments.
Amy M*o*