A question - and sorry for re-starting a tired thread; I've just come across it.
The rules in this country (NZ) allow an aircraft to fly to a higher category, provided ATC knows about it. But how would one go about telling them?
"Request ILS 23L, and aaah we'll be Cat C today."
AFAIK one simply operates to the category one feels like operating to (higher not lower), and obeys speeds & minima for the new category regardless of aircraft type - and who cares?