I agree with AT. If they are now roosting on the roosts and you put the nest boxes lower than the roosts, you should not have a problem.
I just posted this on another thread so I'll just copy it here.
Why would you delay opening the nest boxes? You want them open when it is time to lay their first egg and for a few that can be as early as 16 weeks. That's pretty rare but it has happened. You don't want them to get in the habit of laying somewhere else. Once they get into a bad habit, it can be hard to break them.
If you are going to have problems with your nest boxes, you need to find out before they need them to lay in. They will inspect them and probably scratch around in there. If they scratch out the nesting material, that tells you that you have a problem to fix before they start laying. The solution is usually to put a taller lip across the front.
If they sleep in the nest boxes, wouldn't you want to know that before you got an egg covered with poop? I'd like to know in time to fix the problem. There can be different causes for them sleeping in the nest boxes. One common one is that your roosts are not higher than the nest boxes, but there can be other causes. Maybe you don’t have enough roosts. Maybe some hens are such brutes on the roost that the milder mannered hens look for a safer place to sleep. Sometimes fixing these problems can't be done immediately. The cause is not always obvious. I like to have some time to work with.
