As we all acknowledged when signing our contracts, perks aren't guaranteed to continue indefinitely or to even be offered at all. There are many ways DVC could restructure their benefits program to exclude any past purchasers, whether direct or resale. So no legal issues at all, should they decide to do that.
Whether they would or not is questionable but even in the worst case the only reasonable solution for members who might be unhappy would be to sell their contracts and that doesn't really hurt DVC at all. They really don't care whose name is on the check that pays the member dues each year.