I remember "the rules" about reservation points by thinking about how a non-
DVC option is paid for.
If you use your points to take advantage of any of the options in the Disney Collection (non-DVC WDW resort hotels and the
DCL), the Adventurer's Collection or the Conceirge Collection, DVC must send cash to those places to pay for it. DVC gets the cash by renting out the equivalent number of "point nights" in a DVC resort. Once they've done that, you can't use those same points to reserve a DVC room because the "space" is gone. If DVC allowed you to cancel an option in one of those collections and then reserve a DVC room, - well you can see it would throw the system out of whack, so to speak! They can't rent the same rooms twice for the same time!
Therefore, once you opt to "take the points out of the DVC system" for the Collections mentioned above, you can only use them "out of the system". You can move between any of the non-DVC options listed, though, because if you cancel, DVC still has the cash to apply to your choice.
The Member Getaways (trade via Interval International) works in a similar manner except that once you accept a MG option, you can't use the points for anything in any other program except a MG. That's because when you accept a trade, DVC "deposits" an equivalent week into the II system. That week probably disappears very quickly. In any event, DVC can't take the deposited week back.
You can't cancel a Member Getaway and use the points for anything else outside of DVC because DVC didn't get any cash for the "deposit" (it was a trade of time).
HTH a little.
Best wishes -