I wish you had asked a few days earlier but it is what it is. I move chickens from my grow-out coop to the main coop a few times a year. I have adults in the main coop which complicates it a bit compared to yours. Since I have adults I can't just lock the chicks in the main coop for a few days, the adults need access to lay and it would not be big enough for all of them to spend days together.
I see at least 8 chickens. I don't know the size of that coop. It is well ventilated, I like that. With that green grass you might still have some heat. If it is big enough you can lock them in for a few days to get them used to it. If it is not big enough to just lock them in, can you fix a temporary run around it so they have some room during the day yet can't go back to the other coop?
My method is to wait until dark and move them into the main coop after the older ones have gone to bed. Then I lock the other coop so they cannot go back outside. I open the pop door early the next morning. Sometimes that is all it takes, one time. Sometimes it takes more. So I wait until dark the next night to do it again. I may have to do this for a few nights but it usually doesn't take long for them to get the message. Since my Grow-out coop is a permanent fixture they go back to that area to sleep if they don't go to the main coop, and it is inside my main run so they are really easy to catch after dark.
I don't know how you are set up or where they will go to sleep if you remove that old coop. I don't know how hard it would be to catch them at night. If they sleep in trees it can be really difficult or even dangerous for you to try to catch them after dark.
Have you trained them to come to you when called? If you make a certain call, maybe "Here chicky chick" when you feed them treats or just put the treats in a special bucket or container and rattle it you can train them to follow you as long as you are consistent. It would be fairly easy to lure them all into the coop in the late afternoon or early evening before dark so you could lock them in there. Even if you haven't trained hem to come to you when called, since you feed them in there you may be able to lure them in anyway, it is probably worth a try.
I don't know how it will go for you. It may be extremely easy, it may take a bit of effort from you. But it should not be that hard. It is always possible they will make the switch just by removing the other coop, especially if it was pretty close to the new coop. It is quite possible you will need to come up with a way to lock them in he new coop for a night or a few nights for them to get the message.
Just play it by ear, you should be able to work it out. Good luck!
