Thanks. I'll do that if it turns out we need to change the flights. I did find the Chase travel number to call after posting and was told the credits would go to my family members rather than back to my Chase account.
When logged into the Chase travel portal, when looking at the Trips page, there are three sections available to look at: Upcoming, Past & Cancelled, & Credits. I had assumed the passengers rather than the payer would get the credits, but that "Trips: Credits" page left me uncertain.
The option to make a flight change is also available in the Chase portal and it says in the FAQ that I could change airports entirely (and there might or might not be a fee).
I booked knowing I was insured against an emergency cancellation thanks to the Reserve benefits. The difference in price for the fully refundable tickets was significant enough that Main Cabin was really the best option. If they end up with some trip credits Grandma paid for, we'll just have to plan another trip! :-)