Sorry - typo. I only completed 7 nights - therefore didn't complete the 12 nights.
So yes - had I still been diamond - the suite upgrade would work. But because I am now downgraded to Platinum after 60 days - called Hyatt Hotline yesterday and they said no to me.
The application of a DSU requires the intervention of an agent so I would suspect that when you phoned to add one the agent would note that you are no longer Diamond and not entitled to them.