Place new birds in house as above with the old birds outside, when the new birds are already at roost let the older birds into the house. In the morning open the chicken house as normal. If they are completely free range it will be best to keep them in a smaller area for the first 2/3 days for them to settle down. You will just need to keep an eye on your chickens for the first 48 hours while they establish a new pecking order. Place an extra feeder and drinker in the run.