Actually it could be done potentially. What they'd have to do would be to take your contracts and UNIT's and give you new ones that matched what you wanted. They couldn't just change the UY on what you already have as it's legally tied to a unit (group of rooms) That mean new deeds and closing for each and every contract. Total cost likely $1000-2000 if you ended up with the same number of points and same or similar demand resort ownership. It'd likely be more if you traded in HH or VB and got AKV for example as you'd have to pay the difference in value as wel.
DVC doesn't have to have deeds, had they made the conscious choice not to have deeds or to hold them in trust, this type of change would be pretty simple. My guess is they'd only do this in the future if you bought a min of 100-120 additional points and you still might not be able to get the home resorts you want or already have.
Bluegreen does this exact thing. Generally you have to buy 3000 extra points or buy an extra resale contract from an authorized retailer. They've also voted to allow members to change their UY simply by filling out a form and paying $125 but they haven't actually put it in motion yet. It's easier to do with BG though because your only priority is the week your points are tied to, not the resort you own at. All of my 72K points are tied to Aruba but I get the same chance at everything in the system that ANYONE else does, actually more since I'm a Gold member and can go on the wait list 2 months prior to the reservation window opening.
Marriott will allow you to trade in a week in some cases if you bought from them originally and the new purchase is at least $15K more than the value of what you're trading in. Last I heard they still take their sales commission off the value of what they allow you on the trade.
