My flocks move roosts frequently, usually as the weather changes. Let's say you bump a flock off the roost just ahead of a warm front. They may not be back on that roost the next night, but by the time the next warm front comes through, they'll be back.
On the other hand, and this goes more for public land, if you keep knocking those birds off a given roost they will eventually move on. A couple of bumps aren't going to seriously affect them, but if you keep scaring them they will move away and stay away for some time. I had this happen one year when turkeys took to roosting on one of my ladder stands. It took about 4 times, knocking birds off before they stayed away permanently. You have no idea how scary it is to come up a ladder and have birds sitting on your seat and on the hand rail and all bust at the same time. I thought I was gonna die! You also have no idea how much of a mess turkeys will make on your ladder stand if they roost on it-- BLEEECH!
The bird never came back to roost on that ladder that year, nor any year thereafter. Furthermore, I have half a dozen of these ladder stands on the property and the turkeys have never roosted on any other ladder stand since, going on 8 years. I think the turkeys sent around a memo.