Without regard to the eventual temperatures that you experience, the chickens need ventilation to allow the ammonia fumes to vent out of the coop while fresh air enters. There should be enough of a flow of air to do this. Check the coop as it is to see if the air is clear. If so, that is how much ventilation you need, and will need in winter. The chickens should be protected from drafts. If the outlet and inlet are offset, then the air will not blow through the coop. It will vent naturally, the warm air will rise and exit while the fresh air will enter.
As the weather cools in the fall, the chickens will acclimate to the change as do all animals. They have impressive feather insulation on their bodies. The cold to which they can adapt is less dangerous than the accumulated fumes that can poison them.
Chris