Welcome to planDisney, Katy! Thank you so much for trusting us with your question and I'll be delighted to share what I hope will be some helpful answers as you plan your upcoming visit to Disneyland Resort!
I'm so sorry to hear your mom is unable to travel with you. The Disneyland Resort team understands when plans change, and luckily, there is some flexibility provided in your ticket purchase. Unless otherwise stated in the ticket's terms and conditions, your mom's ticket can be reassigned to another Guest in a couple of different ways. First, tickets are officially assigned to a Guest at the time of their first use, when they are scanned at the entrance and a Cast Member takes a photo of the person using the ticket. At that time, the ticket is formally assigned to that Guest and can no longer be used by or reassigned to anyone else. So even if your mom's name is on the ticket, it would be reassigned to your niece at the time of first entry to a Disneyland Resort theme park.
However, I understand if you would prefer to change the name on the ticket in advance. I have done this in the past because it helps me stay organized and minimizes any confusion among the tickets of my travel party. In this case, you would simply need to call Guest Services using the Tickets line and speak with a Cast Member to request a name change on the ticket. Again, the ticket will be officially assigned to the Guest at the time of first entry, but it is nice to know the correct name corresponds with the Guest using it.
I hope this is helpful, Katy. Please give my best wishes to your mom and I truly hope your family has a wonderful visit to Disneyland Resort. If we can assist with anything else in the meantime, please feel welcome to reach out to planDisney again.
Magic awaits,
Tishra