Well if it was going into flowering stage, putting it back into veg stage could slightly delay things as well cause some stress to a certain extent.
You could keep it at where you got it and then if it shows more signs of flowering then you will have your answer. After all some people run 18/6 from the get go, so if it just flowers instead, I would go ahead and keep em flowering.
Why force back nature? besides you can always grab some clones just so you have a plant to choose from and a fall back to play with more.
clones can come in handy to keep a good strain and particularly high yielding plant thriving especially when using a sog method.
stick with 18/6 cycle for a lil longer to determine if it was the darkness period that caused it to flower.