From the FAQ that's linked in my signature:
>>
~~ MVCD Introduction Timeline ~~
- 6/20/10 - At the MVCD introduction, Marriott immediately halted developer-direct sales of their MVCI Weeks product and offered DC Trust Points for purchase at the US resort and sales sites; and, offered the DC Enrollment option to existing Weeks Owners of the US and Caribbean resorts. Direct sales of the Weeks product continued only at Caribbean and European resorts.
- 6/18/12 - The MVCD program was introduced on a limited basis at the European resort sites, allowing the DC Enrollment option for all existing European MVCI Weeks. DC Trust Points are not yet available for purchase at the European resorts; Weeks are still available at the onsite sales centers.
- 12/27/12 - Developer-direct sales of MVCI Weeks were halted and purchases of DC Trust Points began at the Caribbean resorts.<<
I'm pretty sure that at every Marriott introductory date, only Weeks which were closed and active in owners' accounts prior to the dates were eligible for enrollment. I vaguely remember people being - justifiably - upset because they had resales in process and no notice had been given in advance of the launches. With Vistana it's different because they've given at least some kind of warning, but I wouldn't even try to guess if this soft launch period is a grace period for people to try to get some Vistana resales closed prior to the official launch, especially with the delays that are currently being seen.
Enrollment, though, is a different animal from the annual election to exchange enrolled Weeks for DC Points, and my question is related to elections. Jabberwocky seems to have answered it, that elections can't be pre-set or transacted until after the official launch.