Do you have a run with the coop you want to move them to? That makes all of this so much easier, especially if it is a decent sized run. I assume the coop you want them all in is big enough?
Whether you have a run or not, I’d move the hens at night when they are easy to catch and put them in with the others. Then I’d be out here the next morning before they woke up and open the pop door. That way they can run away if it gets too violent. If you have a run, leave them locked in the run for three or four days. Do this when you can be there at first and observe, just in case it gets bloody. I don’t think it will but it’s best to be prudent.
If you don’t have a run, you’ll need to keep moving them at night until they get in the habit of sleeping in the new coop. I’ve never tried this with two adult flocks that have established territories, but from moving adolescents into the main coop from a grow-out coop I don’t anticipating it taking that long. Locking the old coop so they can’t get in should help. If they don’t go to sleep in the new coop immediately they should try to roost near the old coop, probably pretty close to the door. If they do that, hopefully it will be somewhere they are easy to pluck from the roost.
They will sort out the pecking order if they haven’t already. The rooster should help with that, a good rooster helps keep order in this flock. I know you read about nightmares on here, and they do occur, but most of the time they sort this out pretty easily. Normally this goes pretty smoothly.
You have another problem though. They will want to go to the old coop to lay eggs. If they can’t get in there they will find a new place to lay. That might be in the new coop but there is a real good chance it won’t be. So I’ll offer an alternative, provided your coop is big enough.
Build a pen inside your coop to house the hens. Include two nests. A roost up higher than the nests might be a good idea. When you move them that first night lock them in that pen and leave them in there for about a week. After a week, just open the door to the pen. That should retrain them to lay in the new nests and go to bed in that coop. Just ne down there fairly early the first morning after you open it up to see how it is going.
An alternative is to cut a news pop door, build a run outside that pop door, and partition off a bit of the inside with then nests and roost. This is a way to stretch the size of your coop. Since you are not worried about predators with any of this, you can use cheap chicken wire.
All this will be so much easier if you already have a decent sized run. A larger coop gives you more flexibility too. But even without that you can manage. Good luck!
