thegoofyfamily hit on it but I want to add on.
When you switch, if the new reservation has a higher one night deposit rate, Disney will charge you or request payment for the difference to get your reservation in good standing. It could be that you changed from a lower rate period (summer) to a higher rate period (Christmas). It could be that you changed from a lower cost site (Full) to a higher cost site (Premium).
I do want to point out, though, that if your change works in a reverse way (where the new deposit required is LESS than what was previously paid), you are not refunded the difference.
Those caring, intelligent, compassionate Disney managers are so consistent aren't they?
Bama Ed