But, John... don't forget one thing.
Birds have immunities to the things that are in the area in which they have been growing. But if you move them somewhere else, you have a couple of other factors going on.
1. The stress of the move. Chickens are creatures of habit and can get a bit "wonky" when moved. A stress like that can put a stress on the immune system for a bit of time.
2. The things they were exposed to where they're coming from are sometimes not the same as what's on the land they're moving to.
For point 2, it's good to see how they respond to what's in the new environment and give them a chance to build any needed immunities without the extra stress of being put right in to a different flock in which they will have to learn the pecking order and find their place. A little extra time can save a lot of stress for them.
Not to mention that the current birds also have immunities to what is in their native area - but possibly not the things that the new birds are bringing with them.
So...even if they are all healthy now, giving them a bit of time to adjust to what each side of the equation has coming with them is a good idea. Then perhaps all they'll need to deal with a few weeks down the road is how to get along with the others and finding their place!
Okay.
John