Chickens are creatures of habit. Your little ones are in the habit of sleeping in the little coop. They are also scared of the older hens. It is possible that the young ones, or more likely some of the young ones, will move into the big coop after they become true adults and truly integrate and make friends with the older hens, but I'd expect most of them to continue sleeping in the small coop as long as they can.
Just because they spent some time in the big coop a few weeks ago does not mean much now. They are comfortable in the little coop and the run. They may eventually explore and learn to go into the big coop during the day, maybe even learn to go in there to lay eggs, but right now it is foreign territory to them.
You can block off or take out the little coop and see what happens. I'd expect them to huddle on the ground in the run to sleep at night, but maybe they will go into the big coop to sleep. I suspect you will have to either lock them in the big coop for a few days for them to break their old habit and get a new one, move them from wherever they are sleeping to the big coop at night after they go to bed using as little light as possible, or somehow lure them into the big coop (maybe with treats) shortly before they go to bed and lock them in. I really don't think they will make the move on their own.
I don't know the size of your big coop, the personalities of your hens, your management practices, anything like that. I had my 12 week olds sleeping in the same coop with the adults, so it is very possible your 15 week olds will be OK. Sounds to me like you have done a good job on the integration. If your space is tight, you have an especially aggressive adult, or maybe you leave them locked in there for hours, you may have problems. For the first few days I'd suggest letting them out of the coop fairly early, like just about the time they wake up. That way the little ones are not trapped in tight space where they cannot get away from a big one if they need to. What I expect you to see is the little ones staying on the roosts while the adults are on the ground. After a few days you should be able to have confidence that they can stay locked in there together without serious problems, but I'd be cautious the first few days.
If you are going to leave them locked in there for a few days, you have the problem of what to do with the big girls. You do not want to teach the big girls bad habits, like not laying in the nest or not going into the coop to sleep. In your case, it might be easier to lure them in just before bedtime or move them in after dark for a few days.
Good luck with it. It's probably not going to be as hard as I may have made it sound.