Is it safe to tweak your light cycle times when you flip?

You can easily change, just give them an extra long night to align it to your new schedule. Do not give them an extra long day.

The most important thing is to not cause a short dark cycle if you have already flipped to bloom light/dark cycle. Make sure it is at least 11 hours and uninterrupted, and you can alter your cycle as much as you want. if you are still in vege, i'd extend the light cycle, while reducing power in a proporitonal manner to extra hours applied to get to new cycle. you don't have to worry about confusing the plant in vege phase, so don't give it less light per day, since you can avoid it without repercussions. As long as you are in vege, simply turn the light on at 11pm (an hour early) and let it run to 5pm (or 11am if starting bloom immediately) if on a 18h light cycle, dim current power by 5% (not -5% ... e.g. if at 70% x 95% = 66.5%) --- **** this wouldn't only be necessary if you are really pushing your light to give max DLI (relative to your local environmental variables) and worried you'll give slightly too much light.

All good, my friend! When I want to do this or something similar, I try to give them more darkness than light when it changes to flowering. If I want to keep them in vegetation, I try to do the opposite, giving more light than darkness. In your case, I would switch without any problem, since 18 hours of darkness and then turning on at 9 AM gives them 15 hours of rest before flowering, and they actually like it and stretch a bit. That's my take, not looking for points. Thanks! Happy growing!