As saturn28 suggests, I think a number of people here who seem to think Marriott can't change its reservations program for resale owners are missing a key point. The various POS documents are written in a permissive manner, not a restrictive manner. In other words, they state that an owner may make reservations 12 months in advance. They don't state that an owner cannot make a reservation more than 12 months in advance.
Those MOC documents also say, as I recall, that an owner of two MOC weeks may make multiple-week reservations 13 months in advance. Some years ago, Marriott quietly extended the 13-month reservations benefit to those who any two or more weeks, whether at the same resort or not. However, I don't believe there is any language in the older POS documents that specifically allows that. There is one active TUGger who fought Marriott on that point, posting here about it for several years, and ultimately got nowhere.
Accordingly, if one gets creative, one could imagine many ways that Marriott could add or extend some benefits so as to treat all current owners and those who purchase direct from Marriott in the future differently from those who purchase resale weeks in the future. Based on the POS wording, it might be easy for Marriott to do that.
Even if that's not what Marriott is planning - and I have no knowledge either way - Marriott has obviously had its best legal minds review every detail of its plans.
Marriott has made many changes (e.g., instituting new fees for a variety of services (e.g., for ROFR waiver) and, so far, no court has ever overturned any of those changes. I'm betting that whatever changes Marriott makes here will also withstand any legal scrutiny.
As for whether resale values will plummet, I think Marriott is a lot smater than we are and won't make changes so significant as to allow that to happen.