I would do all chick crumb after the current layer feed is gone, just to keep things simple.Should I do half layer and half chick crumb? Or even more chick crumb? Is there any possibility of issues using just chick crumb? If extra calcium is available on the side?
there’s not much of a price difference chick crumb is 16 for 25kg whereas layers is 11 for 20kg so I can switch all to the chick crumb if it would be better for them
Or if the layer feed is pellets, and they scatter the chick crumb too badly, I would leave a feeder of layer pellets available, but give them chick crumb as a wet mash. Just put some in a dish and add water, then let them eat it. I would try to give them mash each morning, an amount they will finish eating sometime in the afternoon or evening. That way the mash won't sit around long enough to spoil, but it will provide most of their feed for the day. If they have a feeder of layer pellets available too, you don't have to worry about them being hungry after the mash runs out each day, so you don't have to be too fussy about making exactly the "right" amount of mash.
For either of these ideas, I would also provide free-choice oyster shells, so the layers can get the calcium they need, when they need it. Crushed eggshells are also fine as a source of calcium, but the hens need more calcium than they can get just from the shells of their own eggs. So you won't have enough eggshells to provide all their calcium, unless you are also buying quite a few eggs and giving the hens those shells too.