It would be interesting for a fast Z to race in the slowest sprint group where, going on last year's times, it would be quite competitive. To get into the slowest sprint group all that has to be done is to nominate an appropriate lap time in the entry form. If that is done there would be no chance of getting mixed up in the pro racers groups.
For the regularity entrants, as has already been said, the nominated lap time can be changed at any time. Just go up to the timing room and arrange it, simple as.