How to switch to a different time sheme?

Do it in its next off cycle period, so it has a longer lights off time then continue with the next schedule. Also if they are not in a 12/12 photoperiod then it does not matter and can do it whenever you want. Autos don't care about light cycle changes as much as it does not fuck them up as much as a photoperiod.

I'd do the same as papa T

"in either case, you lose the same hours of light over time. you could mitigate that with a bit more power from lights if concerned," -- this is incomplete... that only would be an option if "stepping" via the dark cycle slowly to new schedule. if you go from 12 to 11hours of light, you could give 1/12th more PPFD and maintain same exact DLI each 24-hr cycle. stepping through the light cycle is possible to. stick to 30-60min increments. in this case you could give 1/12th less light hour over 13 hours and it too would provide same DLI. again, i don't suggest this but this is the math and it is true. The effect of a few less hours of light over an entire grow cycle is likely to be so small that this sort of effort is not worth it.

If in flower phase, simply avoid shortening dark cycle. so, extend the dark cycle until you start your new schedule. you can also slowly "step" that direction, again, only if you don't shorten the dark cycle too much or take your steps by extending the dark cycle. In flower, it needs 10+ hours of uninterrupted darkness each day or you will cause a re-vege. To be safe, stick to 11 hours of darkness as a minimum. Not all plants react to the same length of dark as the trigger for bloom phase. in either case, you lose the same hours of light over time. you could mitigate that with a bit more power from lights if concerned, but it probably won't have an effect you can discern with your eye anyway. The only thing you have to remember is that the length of uninterrupted darkness is the key, here. As long as you don't interrupt it or shorten it too much, all is well.

If there is no need to change leave it. I have had many power outages over the years during early flower stages. Some of these outages lasted 24 hours and the plants were fine. Treat it like a power outage flip the lights off at 21:00 then do the switch to 19:00-7:00.