I would put your chances of an upgrade as a gold to be about zero. Even as a diamond, who has not stayed there before, about zero.
Or less.
It's a very diamond heavy property, and they treat repeat visitors with better perks than first time visitors.
I would buy the upgrade if you want it , and not hope for them to offer you one from the suite you currently have.
I've also been told they can't add anything to a prepaid reservation, so that's not unusual.