Hi mate, photoperiod plants are light reactive. What mean is, the plant needs at least 12hrs of darkness to simulate flowering conditions. If you give her 12/12 till flower then switch to 20/4, you are basically doing it backwards and the plant will start to reveg or even hermie. In short, don't do that. Keep her at 12/12 or start at 20/4 then switch when you are happy with her training. I hope this helps, good luck and happy growing