Only certain hotels allow it and it is always via 5K upgrade certs. MVC's charge one upgrade cert per bedroom- so a cat 7 is 35K + 5K for a one bedroom, 35K + 10K for a two bedrooms, etc.
Resorts will often charge one upgrade cert for ocean view, 2 for ocean front.
It is entirely up to the property.
Upgrade offers on arrival have always been cash IME.