Do they usually spread their wings and fly to the edge of the nestboxes? Or just jump with their wings closed?
I think jumping would be fine, but if they try to spread their wings to fly I agree they might smack into a lower roosting bar.
Oh, I just remembered a reason you might not want to do that. Where the roost is now, they can probably walk underneath comfortably, so they can make good use of the floor space. If you lower it, they will have to jump over or duck under, so they cannot use the floor as easily. If they need to...
After looking at the inside, I see a way to make more space between the chickens' heads and the top vents: lower the roost a bit.
If you put the roost about halfway up the front of the nests instead of at their top, it will still be the highest place to sleep (so they should prefer to sleep on...
For chicks hatched in the spring, they molt quite a few times as they grow, but they usually skip the big fall molt.
For adult chickens, that have already been through one winter, they do tend to molt sometime in the fall, so they have nice new feathers before really cold weather arrives...
You don't need to avoid all air movement, just strong wind.
As a basic rule of thumb, if it is not strong enough to ruffle the chickens' feathers, it is not a problem. (That is different from people. For bare human skin, any amount of air movement can make us colder.)
Chickens stay warm by...