Diamond owners tend to have more points even with the 1.6x conversion of HGVC. Putting Diamond on par with HGVC owners to book at same time would flood HGVC as Diamond owners would arbitrage into higher end properties. HGVC owners would never have access to the properties and system that they bought. I believe from comments in TUG from Diamond Owners that they feel the same way about HGVC.
I believe that MAX does not allow unfettered access across the portal, for the very issue that you suggest.
They have to maintain a balance of points/weeks on each side of the portal to assure that owners who are not in MAX have access to reservations consistent with their reservation rights. That points to me that access across the HGVMax portal has to be limited in someway to align with the ownership and usage rights of owners who are in HGVMax. Of course, that's not something they would ever mention in a sales pitch. I'll talk on the DRI side, since I'm most familiar with that operation. To throw out some numbers just for illustration. Let's say that the value of ownerships at a given resort amounts to 10,000,000 Club points, and the Hawaii Collection (trust) holds 7,000,000 of those points. (The other 3.000,000 points would correspond to the point value of deeds owned outside of the Hawaii Collection.) Then, let's say 5,000,000 of those points are part of the (formerly DRI) Club. (The other 2,000,000 points correspond to Hawaii Collection owners who are not part of the Club - this is where resale Hawaii Collection ownerships will end up.)
We're early in the HGVMax rollout period, so let's say that ownerships of Hawaii Collection owners who have joined HGVMax tally to 200,000 points. That then becomes the upper limit for the point value of Hawaii Collection reservations that could be made across the HGV max portal. And if those HGVMax members who are Hawaii Collection members have made reservations using their Home Resort advantage, that should also cut into the Hawaii Collection availability across the HGVMax portal.
If this is anywhere near correct, until HGVMax has a large block of owners, there isn't going to be a lot of access across the portal in either direction. And if they don't do something such as this, the system will be unable to reconcile ownerships and usage rights. A competent financial auditing team would issue a warning in their audit/
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Also bear in mind that home resort early reservation periods still apply. An inventory that is available across the portal will be less than six months before check-in. So owners will have ample opportunity to reserve in their own resorts or collections before the portal opens.
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FWIW - I went through that inventory control issue with a sales rep in an update, and the sales rep agreed with me. But I have zero confidence that she actually knew what I was talking about, let alone that the sales force had been briefed on that during their orientations and training.