Carol, ANY substituting of points requires canceling and rebooking the room, this is the rule. It is not divided up by home resort or non home resort points. I do know that MS has the power to do it the other way but the policy is not to. This is a change from a number of years ago. But like anything else Disney, I'm sure it is inconsistently enforced.
While I'm a big believer in owners getting every benefit if they own at THAT resort (including unit assignments), I don't think that simply substituting the points when all is booked up is an appropriate, reasonable or fair thing to do. Especially given the current way transferred points are handled. Even without transferred points, it would certainly give some owners a major advantage in using points and banking related to high demand times. I can think of several ways to use and abuse this issue, which is why it was changed a few years ago.
I guess I could expand on those thoughts but doing so in the past has gotten me in trouble so PM me if you want more.