If you are shifting to flower, simply extend the lights off period to 12 or 13 hours.
Some argue that a gradual shift will put less stress on the plant, and simulate natural habitat.
But again, if the plant from breader is coming from a artificial enviroment and they shift without gradual shift, this is what you should do.
I've been doing research myself, as i just shifted to bloom
And i found that gradual shift brings pros and cons.
For me personally, the cons of having the extra attention changing the light schedule gradually
And the cons on how long the shifts would take
Did not do justice for the pros of a gradual shift.
One of "Garden talks with Mr. Grow it" touches on this subject.
Here the experts argues that through their research, they found no advantage of a gradual shift.
But again, its trail and error. :)