My 5.5 week old chicks were evicted on an April 1 morning a few years ago. Got down to 18 degrees that first night, and the second. The third day it snowed. Our last snowfall that year was on June 6th. They were fine. I’d wean yours off the heat for a few days if they are still on it, let the room they are in darken slowly as the sun goes down and lighten as the sun comes back up. They’ll hate your intensely and they’ll let you know about it. But they’ll forgive you. Eventually they have to go out. If not now, when? It won’t get any easier as they grow more dependent on artificial heat and light. If I’d kept mine in until western Wyoming spring temps matched conventional charts on chick raising, they’d have been in the house until they were 17 weeks old. Yea yeah, not happening! Now I brood outdoors from the start. Every batch. Every time.