Mark, I'd have thought that it would be up to the individual owner to leave this decision right up until the first race. Once a boat is raced in a series as uni or sloop then it has to compete for the rest of the series in than format. So, in the case of the GC, I'm not aware of any special circumstance that requires entrants to declare in advance which rig they will be sailing? At the time of entry, I guess, most people will indicate their format but really it's not until the series starts that their rig choice is 'set in stone' as it were.