@NORBY, I did something similar by going from 24/0 down to 19/5. I just did it in small steps going down 15 minutes at a time. Didn't see any adverse effects. I did 24/0 through veg to see if it would make a difference. Believe it did as the plants seemed to be a little bit larger, but no definitive proof.
Now I split the difference and do 19/5. The most common schedules are 20/4 and 18/6.