The feathers will likely grow back in during the next molt. You can use a hen saddle to protect the back until then. And raising the protein level in the diet can help with feather growth. Switching to a flock grower or raiser feed will have more protein than a layer feed, also there are feather fixer formula's available that have higher protein (in the 18 - 20% range). If you switch to a non-layer feed, make sure you are offering oyster shell in a separate feeder, free choice all the time. The non-layer feeds don't have enough calcium for laying hens, but they will take the shell when they need it. Those feeds are better for mixed flocks with roosters anyway, the roo's don't need the extra calcium and getting too much over the long term can cause health problems for them like kidney problems and gout.