This happened to me earlier in the year. When the meanie left I was wondering how the flock dynamics would change. I have 12 girls. I was worried that maybe they'd be more vulnerable to predators. I have lost 1 bird to a hawk, but lost 2 while he was there, so that hasn't changed much. What HAS changed, is that the girls pay attention to me now. And one hen has become the watchdog patrol. It's funny- when I let the girls out of the run to roam, she stands and watched with her tail straight up and gives the warning call when there's an overhead threat. Otherwise, she lays and acts like a hen. My girls have lots of spots to hide if there is an overhead threat. There are shrubs, and we have put a "coffee table" made of pallets and plywood halfway between their coop/run area and under the porch where they dust bathe. We put a water dish under it.
Every flock is different, so I can only tell you what happened with me. You might have a few squabbles while they figure out who's boss now, but I think they'll be fine.