The official documents, other than those for Riviera, specifically provide that any member (new or resale) shall be able to reserve DVC Resorts that are part of the system other than the one the member owns. The documents do provide that such right can end if certain unexpected events occur such as the DVC entities gong out of business, the resort is withdrawn from the DVC system by a vote of the members. Moreover, the right can be suspended (but not done away with) due to certain events, such as a disaster that destroys much of a resort, but the right then needs to be restored when the special circumstances creating the suspension are overcome. Moreover the events that can lead to suspension apply equally to new and resale purchasers and Disney could not suspend the rights of resale purchasers without also doing the same to purchasers from Disney.
So if Disney actually obeys the Public Offering Statements (POS) and contracts relating to the resorts (all of which it created), then the answer is no, Disney cannot punish resale purchasers of the original 14 resorts by taking away their rights to reserve other resorts.
The problem, however, is whether the prior resort POSs and contracts mean anything to Disney. The same provisions in the POSs and applicable contracts of the original 14 resorts that require Disney to treat new and resale purchasers the same when it comes to reserving resorts other than the one the member owns, also specifically provide that when a new resort is added to the existing DVC system, the same rules shall apply, i.e., the new resort added to the DVC system has to have same right to allow members (new and resale) of other resorts to reserve the resort. In adding Riviera, DVD apparently decided to ignore the controlling provisions of the POSs and contracts applicable to all the prior resorts, which prohibited DVD from adding a new resort to the existing DVC system that did not allow all members, new and resale, to reserve the new resort.
So the answer to your question is that the provisions of the POS's and applicable contracts of the original 14 resorts should prevent Disney from taking away the rights of resale purchasers to reserve other resorts except to the extent that such rights can be suspended (but not done away with) as to all members (not just resale) due to certain events. However, Disney has already chosen to ignore those provisions when it added Riviera to the DVC system. Moreover, during another dispute last year involving attempted changes to the
point charts raising studios and 1BRs almost year round (later withdrawn), Disney claimed that it believed itself not bound by prior representations it had made at time of sale of the older resorts. As a matter of practice in the last 10 years, when taking away things from resale purchasers, Disney has always applied the change only to resale purchasers that purchase after the date the new restriction has gone into effect. That is actually its main methodology that helps it avoid lawsuits from existing members. You may be able to take comfort in that practice which protects reservation rights of resale purchasers of the original 14 resorts to reserve other resorts, but based on the acts of Disney in relation to Riviera and the point charts, and its attitude of believing itself not bound by its prior representations, you likely should never assume anything is completely safe.