IHG rolled out their milestone rewards in 2021 and there have been frictions in terms of implementing some of them, making them uneasy or even impractical to redeem. As a goodwill gesture IHG extended the expiry of the F&B rewards, but nothing was done to the confirmable suite upgrade ones.
As reported by a friend, followed by many others, you can apply for an extension of the CSUs through IHG’s customer service.
Give it a try if your CSU is due to expire soon. Note the language however – it is a “once-by-lifetime” extension, so only apply if you are sure about using it before it expires again.