does changing the light cycles of auto flowers stress it?

Nope... autoflowers are pretty rugged that way - they have a will to live and grower-be-damned! LOL! 18/6 is an ideal schedule for light and dark - but last winter I had to run my lights 24/7 to keep the temps up in my tent and my girls didn't bat an eye... This year, I'm trying them on 20/4 and again, they don't care although I DO see them start to sag just before lights out as if to say "Mom, I'm tired, I need to take a nap" ... LOL! But the overall message here is no, they are not stressed by the 4 different lights, changing times or anything you've put them through! Enjoy the journey and good luck!