What to do with older chicks once new ones arrive?

Because of the size difference, the day-olds will be pecked on by the larger older birds.
If you do not have a coop set up outside for the older birds, then you'll have to have two brooders to keep them separate till the new ones get larger.

 

You probably should make it so the older chicks don't come into contact with the new chicks, so they don't get pecked or stepped on.

 

For best results you will need a separate pen, or fence off part of your run for the younger birds. Let them share a fence line for a while so they can all get used to each other before they actually mingle. You don't want any of the younger birds to get severely pecked and you don't want any of your older birds to get it in their heads to peck!

I raise chicks from the age of 5 weeks in a grow out pen alongside my older birds. They stay there until I start letting them out into the pasture at about 15 weeks old. By that time everybody is well used to each other and pecking is extremely minimal.

 

I was going to post the same question! I have 3 eggs incubating right now but it was spur of the moment decision. We have been planning on picking up our chicks tomorrow. We've been waiting for the local co-op to get them in. So these will be a few days old. But my eggs are not due to hatch for another 2 weeks. I wasn't sure if we could put them all together or not.

That is super cute that your chicks go into their tub in the evening all by themselves.

 

A friend of ours keeps her younger chicks (once they're older and ready to join the coop hens) in a hardware cloth pen in the chicken coop for a few days and then when everyone seems acclimated to that she opens it a little so the younger chicks can go in and out but the bigger hens wouldn't be able to. That way the little ones can go out with the bigger ones, but have a safe place away from them. She keeps food and water in the little chick's pen, too, just so they have somewhere to eat and drink without the big girls bullying them.