It wont put it back in veg, but it has a VERY HIGH potential to hermie.
People do "dark period" stress right before harvesting, because the plant puts all it's energy into trying to save the plant by producing stronger flowers. With a plant not very close to being done flowering, the plant will still want to save itself from the stress of excessive dark, and will accordingly make male flowers to try to pollinate itself and make seeds.
You NEED to buy a timer. If they are autos, 24hr is not recommended under flower but is better than complete dark.