If at all possible it would better to build a completely new coop adequate for your flock size(current and future) and climate....and save the tiny coop for isolation(more chicks/broody/hospital.
So the older chicks are in the coop shown and the youngers are still inside or.....?
It can be done, carefully, with a good setup.
What kind of housing and heat do you have for them?
Pics would help here.
I had some days old that I merged with 4-6 weeks olds.
I split the brooder with a wire wall and separate heat for the tinies.
The bigs wanted into the tinies space, the tinies...