hello
I usually first think about my schedule and when I'll have time to tend to the plants(work, school, sleep gets in the way), environmental factors like if it's winter i try to run my lights more in the evening to keep them a little warmer when it's coldest. Stuff like that.
Then i just flip them. I don't ever run a 24 hour schedule so the transition from an 18/6 isn't ever so drastic that it pisses my plants of when they go to 12/12. Also if your environment is in good shape, temp humidity etc, and they're healthy going into flowering it should usually be a smooth switch.👍