I assume you don't have a car available and will be relying on Disney transportation to get you to/from the resorts, etc. In that case then Bell Services should be able to help with the transfer. It won't be immediate so you may want to keep a day-bag full of stuff you might want the day of the resort change.
We just booked a Fri & Sat night cash stay online through the Disney website and will be transferring to AKV the following week. We will have our car with us so getting moved isn't a problem. I'm not sure how the bookings work when you call but we had to leave a deposit online to reserve the booking and will pay the balance at check-in.
Good luck!