Yep you can just go from 24/0 or 18/6 to 12/12 for flowering. If you are using a HID(MH or HPS) light Id recommend 18/6 for vegging. 24 hours will create some huge plants and an even huger electric bill. If you are using those CFL's 24 hours is good.
If you got a ton of space and deep pockets you can run the HID light 24 hours if you like. And can go right to 12/12 for flowering. You dont need to gradually change the light cycle unless you plan on transplanting outdoors. I grow outdoors and start mine under a shoplight for seedlings and a 250w HPS for a little veg time, I use 15/9 right before I transplant. When they are sprouts 24 hours under a flourescent shoplight with 2 grow bulbs in it for about 10-14 days then under the HPS for 18/6 at first and I gradually bring it down to 15/9 before transplant. Indoor grows, you really dont need to do any of that though.
After Vegetation is flowering, nothing in between.
Happy Growing homey
