Book what you want now. If the property drops a category or two, you'll get the new rate. If it goes up, you'll keep the current rate. If you see a different property that dropped a category and is now more attractive, cancel your reservation and book at the new property.
We will get advance notice. Even that mid-year change in Budapest - cat. 2 up to 5 - came with lots of advance warning.
"Book what you want now" is fine, but I certainly wouldn't waste time with speculative bookings that might be forgotten about.