OK to switch from 12/12 to 18/6 for auto in flower?

depends on intensity of the light you are using... as long as u don't provide more light than the plant can handle per 24 hour cycle, more is better. google for reading material on -- DLI, VPD... and relationship of temp and atmospheric co2 with photosynthesis to understand why "max" levels of light per plant/garden fluctuates relative to other environmental factors. More is not always better. there is variability in genetics, but these plants evolved to be similar in this respect. cheaper the light, the more heat it produces, the more watts you can provide per square foot... this is why watts are a horrible measure to use.. learn umol/s ... learn PPFD/DLI etc... this is an apples to apples way to assess such things... temps. rh and atmospheric co2 will still vary and have their impact, of course. figure 40DLI is maybe a bit over what you can provide given atmospheric co2 and loosely controlled environment at least avoiding extremes and within a range at minimum suitable for plants to survive. It's good to be 5-10% over what you need... you can mitigate dimming of diode over 3-7 years (depends on quality / how hot they were driven in that time) L90 under optimal conditions takes 50,000 hours before it dims to 90% of original... 6-10 years of use, but cheap lights will never go that long before dimming faster even when using samsung lm301 diodes. (watts per diode is key... closer to 0.25w per diode, the better quality the light is, the more efficient it will be). fine tuning requires observation... Resulting internode lengths and/or visible light damage can be a guide... adjust and take some notes until you have it memorized, cuase it'll be similar next time. from mature vege on it can handle same DLI... whether over 12 hours or 18 matters little to the plant.

I was hoping I could increase the lighting schedule and not make them stresses. Thanks for the answers and support!😃

Yes switch the light over to longer, just don't exceed a 20/4 schedule. They need some rest, plus it is easier on equipment. Good luck and happy growing 🌱

An autoflower is completely independent of the day/night cycle, managing to thrive even under 24 hours of light! If you need to change the light, I recommend that you do it when the lamps are off.