It seems that HPP runs more like the Westin/Sheraton Flex products do more so than how Marriott handles DC. Westin/Sheraton Flex owners can only book weeks inventory inside of 8 months. Weeks owners with StarOptions can only book Westin/Sheraton Flex inventory inside of 8 months. That is if Vistana makes the inventory available. However, with HPP it doesn't sound like there is any cross over between weeks (HRC) based points and HPP.
Hyatt is likely to create an exchange company that will allow HPP and Welk Platinum Points (WPP) to trade between each other. This is kind of like the Marriott DC exchange company (not the trust). There may be multiple ways to get into this exchange company. Perhaps simply owning HPP or WPP, either now or in the future. Perhaps some kind of enrollment fee, available to everyone, even HRC weeks owners. Perhaps require the purchase of additional HPP or WPP in the future. It would be very difficult to merge the two trust together. I think that is what is causing a lot of issues with Marriott trying to integrate Vistana into the Marriott program. Vistana has trusts out the wazoo (Westin Flex, Sheraton Flex, two Nanea trust, multiple trusts at WSJ). Then there is also Westin Aventuras in Mexico. Merging or removing inventory from a trust is messy. I suspect what they would do is any new inventory they add in the future would go into HPP and there is an exchange company setup where all inventory from both trusts is dumped in and all HPP/WPP owners can access it. If there is some kind of enrollment and not everyone is automatically allowed in, then it will work more Marriott DC exchange company and they would move inventory into the exchange company when someone books between systems.