After switching your light schedule to 12/12 your plant will go into mini-stage called pre-flowering or “the stretch”. This only normally last a couple of weeks, but your plant will pack on the most height here. When buying seeds, most companies will label the strain under one of three heights and this will help you estimate how much you plant will grow in pre-flowering.
Short - Usually indica and autoflowering strains. The plant will probably only at most put on about half the height it is now. So if it’s 12 inches then expect it to put on 6 inches more in pre-flowering.
Average - Usually hybrid strains. The plant may potentially double its height. So a 12in” plant may become 24in”.
Tall - Usually sativa strains. A plant may double or triple it’s size. So a 12 in” plant could reach 36 inches after it’s pre-flowering is over.
You will see “pistils” appear in pre-flowering, but you will not see flowers forming until the plant has stops stretching.
There is also a theory (hypothesis) out there that you can essentially skip the pre-flowering stage by giving 24 to 36 hours of darkness before going to 12/12. Cannabis plants have two group of receptors for detecting light changes: Phytochrome Red and Phytochrome Far-Red. When the lights are on in veg these two are balanced, but when the lights are off Far-Red receptors start to turn into Red receptors.
Once the ratio between Far-Red and Red receptors become askew enough your plant starts to flower. The thought is if you give 24 or more hours of darkness the Far-Red receptors will convert to Red receptors faster than they would have under normal 12/12.
I’ve never tried this, but the idea seems sound and it could help growers whose plant grew a little bit too big in veg for whatever reason.