Thanks for your concern, PremEx, but I had already checked with Gold Passport services in HK and they confirmed that I will retain Diamond as long as I can fulfil the 4 stay requirement effective immediately.

In the letter I received, it also states:

"If you are already a Gold Passport member, just call HYATT on +852-... or your nearest Hyatt Reservation Centre to register for the above offers by quoting offer code ... and your current GP number. "

Hyatt will also ask for your Marco Polo number, and note that it's not just Asiamiles. You need to be Diamond at least I think.