Changing an autoflowers ammount of light mid- flower

My autoflower has gone into flowering, little white hairs are popping out of everywhere, but one thing is incorrect is what i'm doing i've had here on 12 /12 in the window from week 4 to now which is roughly 5 weeks later, she's only a foot tall in flower, i know iknow. should be on 18 /6 20 / 4 or 24 / 0, my question is, would it stress the plant considerably to put it under 18 / 6 or higher suddenly?

Absolutely! Keep it as is and learn from your mistake.