I'll assume by your worry that it is NOT an autoflowering plant?
Yes, you can revegetate your plant by putting it in 18/6 and keep the vegetative nutes going.
The plant will, over a week or so, start growing some weird looking leaves, then normal leaves, and that'll tell you you're vegging again. Don't cut off the flowers, just leave them.
You may end up with a very bushy plant by doing this.
Your lower fan leaves are yellowing, a bit early for natural senescence; have you fed any nutrients so far? What soil? What nutes? Anything else you've been adding? What are your watering habits. pH?
You'll get more and better help faster if you provide a lot of this type of info. I think there's a sticky on what info to post.