I'm waiting on replacement parts for our new coop and by the time it arrives I'll put the new girls in the new coop first to claim territory so the old birds don't chase them around the old coop - not likely that will happen but it will be nice for the new girls to be adjusted to each other before facing the established flock.
Just thinking out loud since I think you know I have only the girls I got all at the same time 3 years ago as day olds so I am inexperienced in such things:
Would it be beneficial to not put ANY birds in the new coop until you can do them all together? I'm wondering if having them all a little off kilter would be better than having some with "claim" to the space which is then "invaded" by others. They are going to work out the pecking order regardless and if the new birds may end up on the bottom since they are juveniles, might it be easier if they aren't trying to defend what is "rightfully theirs"?
I'm sure those with experience introducing new birds to old will have good thoughts on this.