can i change the lights back to 18 hours after plants start to flower?

I am growing hydroponics and i just started my 12hr 12 hr cycle. i was just wondering can i change the lights back to 18 hours after my plants start to flower?

You can but it will take area weeks for the plants to start to veg again. I recently did the same thing with one of my dwarf plants, it took her about 3 weeks to turn back and for new growth to start coming.