You should lock the chickens in the coop for a few days, so they "get" that it's home, it's safe in there, it's where they sleep at night, and then you don't have to worry about supplemental heating.
When I first put my chickens outside full time, I'm dealing with integration issues, so I have grow-out coops with temporary runs inside the main chicken run. But I do lock them in that grow out coop for a few days so they KNOW that's the sleeping quarters. Then I let them out into the temporary run for two weeks to let the older chickens get used to them. After that, I remove the temporary fencing....
As they grow up, they do some coop shifting, in that they decide - or not - that they want to sleep in the Big Coop. I have several "grow out coops" that are occupied at night by the chickens who grew up/out in them. Some of them have moved to the Big Coop, most have not.
So I just recommend, for your flock that is all the same age (right?) you just lock them in the coop for about a week. And maybe remove the outside roosts for the winter... not really sure about that, though...
Hopefully other members will chime in with their recommendations!