If they were mine, I would get them a feeder and keep it full of 3/4 layer pellets or crumbles and 1/4 meat bird. Give them a little corn or scratch, but not first thing in the morning, and not more than a cup or two to get them eating in the afternoon. One other thing I would do (and do) is add Diatomaceous earth to their feed (2% by weight - I put a mason jar in a bag), and Raw apple cider vinegar to their water we put a good splash in a five gallon bucket. I don't know why, or how, but the two really make a difference.
The biggest thing is that it will take some time. I am not sure where you live, but I would also not provide ANY source of artificial heat. They may get some protection from a good fat layer, but most of their protection comes from the down layer. If they are protein deficient (and it appears they are) they are going to be building bone, fat reserves and feathers all at the same time. They are remarkably resilient and adaptable animals. Take a picture tomorrow, and take another in two weeks and see the difference. You really can't tell by looking at them every day.
This is just what I would do, one thing for sure is I would not take any drastic steps, I would provide what they need, they will do the rest.
On edit - after thinking a while, I have to change my answer...
They have been on a low protein diet essentially and switching over to a high protein diet may shock their digestion a bit. So with that in mind, If they were my chickens I would do the following.
Keep them on the layer ration for a week, then go to 25% meet bird 75% layer. then 50-50 for a week, then 25% layer 75% meet bird for a week. I would keep them on the 25% 75% until they look like they have filled out, then keep them on that for another week or so, then reduce back to just layer pellets, but I would reduce the ration at twice the rate of the increase so you don't reduce laying. At a two week stretch between reductions, you should be able to notice any reductions and be able to correct easily and with minimal shock to their digestion.
Just my opinion and the course of action I would take if they were mine.