Not sure how I missed it yesterday, but I just saw there may be a clue to the answer to your question in the ABOUND news release in this quote from MVW President John Geller:
"We are incredibly excited to debut Abound by Marriott Vacations, which is our gateway for enrolled vacation Owners to embark on exceptional travel adventures around the globe, helping them make a lifetime of meaningful vacation moments with those they care about most," said President of Marriott Vacations Worldwide, John E. Geller, Jr.
Note my highlighted text in the quote. He specifically said "enrolled vacation Owners". So, I would interpret that to mean that existing enrolled MVC owners are automatically enrolled in ABOUND, since they said there would be no fees for the new program other than the annual club fee all enrolled owners pay. The key outstanding question is what will be the enrollment process on the Westin/Sheraton side? I think we can probably assume all direct sales will be automatically qualified for enrollment, but how will Westin/Sheraton resale owners become eligible for enrollment? I suspect it may be via the same path that an unenrolled MVC weeks owner can become enrolled today - by buying a defined number of ABOUND ClubPoints direct from Marriott. Historically in the MVC world, that option was only offered during certain promotional periods that tended to happen once or twice per year.