Light transition to flowering... How??

There is no need to transition gradually, you can just go straight to 12/12. Some do give an extended dark at the start, in the belief that it will "kick-start" the hormonal reaction to flowering (which is triggered mainly by the length of the dark period). An extended dark transition and a straight flip to 12/12 both work, the only issue is if the extended dark makes them flower a little quicker or not.

Another possible reason to give an extended dark at the start is just to get the lights onto a schedule that works for you.

 

I've only a few grows under my belt, both of which were switched from 24/0 and 18/6 to 12/12 immediately. No stress that I could tell, and if there indeed was any stress, I wouldn't really know how it would manifest itself. Hermies maybe? I would think that any stress related to lighting would be a consequence of inherently bad lighting design or schedule, not the switch.

I have indeed allowed for longer periods of darkness as Toasty alluded to simply to get the 12/12 light cycle closer to my own schedule and to deal with the occasional power outage which throws the light timer off. I don't want to have to work/water/trim the plants at 1 in the morning, for example.

 

I would tend to agree with the other posters. On both of my completed grows so far I've gone strait from 24/0 to 12/12 and both times I was able to sex in 5-7 days and should be able to sex the ones I put on 12/12 monday tonight or tomorrow night. so if theres is stress I cant see it as being much at all. No hermies or slows in growth for me...