Luiz,
Strictly speaking, these days anything built by Corsair is now called a C-XX, even if the boats are unchanged from Ian's original designs. So, F-28r is now C-28r, F-24 is now C-24, etc. If your boat was built before that change the name it's still going to be referred to as a "C" boat. The only models not being renamed are those out of production, ie F-27 is still an F-27, F-24 MKI is still that. The sail insignia is changed to reflect this as well.

The 31 went through a couple of iterations originally, some being built in Australia. Up through the F-31r Ian had input, the new 1D updates are Corsair's.