you do not have to go to 12/12 to determine sex as they will show preflowers on an 18/6 light cycle by the time they reach a certain age wich can vary but is usually somewhere between 3 - 7 weeks old.
if you must go back to 18/6 would probably do it now or you could also take clones now and continue to flower the plants and start the clones under 18/6