Yeah, DVC rentals can be problematic. There are many moving parts to a DVC reservation including availability, point ownership and the willingness of owners to make changes. Most companies simply avoid the whole thing and make changes difficult or impossible. It isn’t just the DVC Store, it’s
all of them.
I would contact them one more time and ask if they would be willing to modify your reservation if a HA studio comes available. It’s the same resort and same number of points but it would require the owner to call DVC and make the change. Availability changes all the time, especially as your check-in date gets closer and closer so there may be a availability later.
If they say “yes” then you can occasionally check. There is a website that shows availability but I’m not sure if it covers HA rooms. I’ll dig it up once I’m on my laptop and not on my phone. (Edited: here it is and unfortunately it doesn't show handicapped rooms:
https://tools.dvcvacations.com/points-calculator )
If they say “no” then you can request a ground floor or one of the few buildings with elevators for medical reasons. You are
very likely to get your request, especially if you are there early and they can swap someone else arriving later out of a room you’ll need.
I also want to say that I have personally dealt with DVC Store aka The Timeshare Store twice as a DVC owner. I bought a contract from them and I sold a contract through them and they were wonderful and professional in every way. They are not evil, they are likely just stuck between a rock and a hard place trying to accommodate you and keep their DVC owner happy.