It's less likely to root the further into the flower period you get. Although, I have heard of ppl on here cloning like 6 weeks in, and it worked. It'll just be harder.
I revegged a plant once. I flowered it, cut off most of the buds at harvest, and then went back to 18/6, and it nearly died, but she came back after like 3 months. I took clones from her, they rooted, and I'm harvesting her a second time for good this weekend.