I've never dealt with the box store, I'd simply call and ask them. The CNDs aren't that unusual, they should be able to answer very quickly/easily.
She already paid in full for a 2024 cruise? That's early, but OK, whatever. I think to use the CND, she'd have to cancel. Again, she should call whomever she booked with to ask.
Be aware, if you are booking a balcony or above, 6 months or more in advance, you can use 2 CNDs. So, if you have two and are giving one to Mom, but you are looking at the same 2024 cruise, you should be able to use both of yours on your cabin. This is what's called the double up promotion and it generally is always available.