One more data point to share:
Got Diamond status match for both me and wife, via HHonors gold promo. I was downgraded to Plat just before my arrival at Hyatt Waikiki and Hyatt Kauai. For both places, I was using a mix of super cheap matched rates (140 at Waikiki) and (180 at Kauai) and I never stayed at Hyatt before, ever. So they didn't want to give me the bennies.
But my wife's account was NOT downgraded and we switched the reservations over to hers. Worked well---we got club access and nice rooms.
As an aside, the Hyatt in Kauai is very nice but more oriented to those who like to laze around. (nothing wrong with that if that's your sort of thing) Club breakfast was decent---wish I figured out how to get to the buffet instead to not deal with the (other) riffraff (like me )