Follow-up from a month ago. I called Disney yesterday and was successful in getting my credit applied to new tickets (thanks again to
@CarolynFH). Here is what I learned during the 20 minute phone call:
>Old tickets were 2-day base tickets for two adults, purchased in January 2021. I had $550 on hold at Disney. Price for new 2-day base tickets was $668. Roughly a $60 per-ticket increase over 4 years.
>The credits had to be used
only for the person for whom the ticket was originally purchased, and the credit will only be applied once (no using some of the credit for this trip, some for another).
>"New" ticket had to be for the same media or something more expensive. I didn't ask if I could use it for a cheaper ticket and forfeit the balance. [Knowing Disney, I bet I could

)
>I upgraded the tickets for both of us to 3-day base tickets. I'd already "done the math" and the numbers Disney gave me were exactly what I calculated on my own.
>I was able to use a gift card to pay for the balance due on the new tickets. Also, the new tickets showed up in my MDE account while we were still on the phone, already linked.
The entire transaction took a little bit of time, but I spoke with two different very efficient, very lovely CMs. It all happened smoothly. They both mentioned that it was most helpful that I had the original ticket number, as all the actual info was on file. All in all, it was a good experience (because I'm never sure with Disney's CMs and IT). Whoosh... it's all done, everything is ready, and we leave for Disney World in about 48 hours!