You would have to rescind and
DVC would do a new contract. That's easy to do with a fax and I'd also call.
You have this one small 10 day window to do this or your locked in. When there's any question at all I 100% recommend taking advantage of that ability and then taking a little time to examine how you want to use DVC and review if owning is even the way to go. You're lucky to have realized you may want to go a slightly different route while you still have the ability to change it up!
Then, once you have a plan your comfortable with you can either call and do the purchase direct, buy resale or a combination of both, or even not to buy but continue off site, with mods, or even renting.
If buying and you do think you'd like BWV you might think of buying that first and then adding on the poly direct after you close on the BWV. Direct has all the UYs available at the Poly so it's simple to match up to an existing UY but when you're having to match up a resale to an existing UY then you become restricted to whatever contracts people are selling in the UY at the time you're ready to buy. There's usually at least 2 or 3 UYs that will be optimal for a buyer so if your not already tied to one you'll have a larger pool too look at. Then DVC will match that UY for the Poly points.
Poly will be there for quite awhile to buy. You could re-initiate the properly sized purchase tomorrow, next week, next month or next year.