Where the credit goes
The property credit is granted once per stay rather than per night, and the useful decision here is to spend it at Il Palagio, the hotel's Tuscan restaurant, which holds a Michelin star. The spa is the other sensible use. What it should not go toward is breakfast, which the program already covers for two in each bedroom.
Eligible charges are set by the property rather than the brand, so we confirm what applies to your dates before booking rather than quoting a rule that may have moved.
What the upgrade realistically does
Florence runs at very high occupancy from April to October, and the upgrade is subject to availability like everywhere else. The difference at this property is that a category change is unusually consequential, because the rooms are not variations on one another. Moving up can mean moving building, gaining a ceiling fresco, or gaining a view of the garden rather than the courtyard. In the low season the upgrade lands more often than most and is worth more when it does.