A policy like this would not be shared simply verbally. It would reasonably be written in documents to which we neither have not need access.
You can't book three nights in the middle because there is an existing reservation for six nights encompassing those three nights.
You could cancel the six nights and hope the three nights stays available long enough to book that instead. You agreed to six nights. Between when you booked and when you try to amend, Disney could have booked someone else for those same six nights - but didn't due to your reservation.
They were, reasonably, expecting you and your money for six nights. Now you want to drop nights one, two, and six. That's three nights of lost income because they likely will not have two other guests wanting those three nights.
Got it. You don't like the inconvenience of sticking with your agreement with the property. Don't stay onsite.