So if I got my auto on a 20/4 light schedule, what...

Ok here's an answer that's a bit more in depth based upon my experience. Bottom line is it comes down to the strain. Some with solid genetics like Mephisto, can take 20/4, and I've even seen growers do them at 24. I'm growing out a few from a not well known breeder of autos. And their auto actions were weak. They started but they weren't spitting like they should have been. Switched to 12/12, kept the nutes the same, and bam! Spitting flowers. I think it comes down to the ruderallis part of the strain. If its bred out too thin, it might not have the kick it needs to be an "all in" auto. Genetics does matter.

Hey Growmie, Changing the light cycle would just make the plant start flowering. Hopefully, I answered your question and remember Happy Growing and Free your mind one puff at a time.