I just go in for the photoperiod switch. I use to a while back place in dark prior to flowering but there was no scientific fact that supported what I was doing made the photoperiod change less stressful on my ladies.
How healthy are your plants, to be on the safe side you can give them veggie nutes for the first couple of waterings, they will stretch and it won't hurt. If you go into flowering lite on nitrogen it won't be easy to stuff it down there stems later......
The nitrogen will feed the stretch,and peeps will transition into a bloom schedule.Some growers use a single nute ratio throughtout entire grow for ease.