You might want to re-pot... how many gallons is the one you have now? You should definitely flip the lights now for flowering... the plant will take its time if its not ready anyways.
Hell yes! The average height of my gals going into the flowering room is 18-24 inches.
They finish around 40-48 inches, right where they get the light best.
"Should i wait a few days...". Only wait if you want them to get bigger. As I mentioned, my plants
often double in height after going to 12/12. That tells me the plants are able to grow while they flower.