Actually you can switch the light cycle around in flowering but you pretty much have to coax the plant into changing. Besides, the time it takes for it not to affect the plant is more than the whole flowering cycle. I tried it awhile back due to Texas weather. What i did was extend the dark cycle 1hr every few days. At 16hr dark i increased the light by the 4 hrs. Let 12/12 run for a few days then did it again. I was only shooting for an 8hr shift in cycles which took four weeks. The plant continued to grow lke a monster. I did a quick switch (24 off then switch) and got hay. Now i'm thinking that a 24 off, 4 on, then +4 every 4 days may be a better option. I'll have to try it this summer.