At 8 weeks old they should be fully feathered and able to handle the cold. HOWEVER, you can't just take the heat away!! They are used to the temperature. I recommend dropping their heat down to 70 degrees now and holding it for 2 days, then dropping it down to 60 for 2 days...then, what are your outside night temps right now. You will want to get away from the heat lamp soon because it's very unsafe in their coop! Coop fires are very bad indeed!
Welcome to BYC. Glad you decided to join our flock. Bridebeliever has given you some good advice as heat lamps are responsible for most coop fires. I would wean them off the heat lamp as soon as possible. Make sure that their coop is draft free, dry, and well ventilated to prevent moisture from building up inside of it. Feathers are wonderful insulators and moisture is a much greater danger than cold. If you haven't done so already, definitely check out our Learning Center at https://www.backyardchickens.com/atype/1/Learning_Center. There is lots of useful information there. Please feel free to ask any questions you may have. We are here to help in any way we can. Cheers.