Can you get extra wire/netting and bend out the divider so it makes an L shape, to give the hens access to the line of boxes? Like this, bending 90 degrees at dotted line:
Take out dividers, maybe saw off the lip that's in front of the boxes that are being converted, and then it's just open...
Do you have a photo of your coop so we can see how you're planning to split the area? Maybe we can figure out a way to still give the hens their preferred box?
I don't know if chickens would associate new chicks = can't get to box = bad!... not sure if they logic things out that way.
They might need to just live with the fact that they can't get to the preferred nest box (have they ever used any of the others at all?)
How long they'll need to be separated: maybe 1-2 weeks, depending. For me, ideally, I want the older birds to lose most of their interest in the younger ones...