This is just my guess, and I base it on the fact that I don't see many current owners wanting to necessarily commit to another full 50 years ....
.
7-10 years prior to expiration Disney will survey current owners of the expiring resorts to determine their desire to remain
DVC members.
This will include evaluating interest in different scenarios such as a 25 or 30 year "Extensions" vs. a 50 year Renewal.
Think of "Extension" as just a shorter length new lease and whichever way DVC chooses, it would be deeded as a separate new phase to avoid problems with those who chose not to continue ownership.
For rollover simplicity, I would not expect there to be any changes to
points charts, especially if they go the extension route.
However all the quirky new rules and restrictions that we see for the newer resorts would be added to these new contracts.
It is even possible that the two phases would have an overlap use agreement which could help alleviate some of the end of lease point jam up. Complete room renovations could be completed during the early years utilizing the surplus of inventory from those that ended their membership.
As for pricing, I would expect an "25 year Extension" to cost 65-75% of the "New Construction" rate while a 50 year renewal for existing owners would be more in the 85-90% range.
Of course Disney's surveys may show that it would be more profitable for them to close down the resorts and not keep current members locked in. While I would find that surprising, it is a possibility.
JMHO