I'd get them breeder feeds for a few months at least, to build them up. Otherwise you're risking them dying not too far in the future. Reproduction is a serious strain on females, it can be lethal to healthy females never mind such starved ones.
Layer feeds alone aren't high enough in the general spectrum of nutrients to ethically feed to starved pullets. To put this in perspective, you've got the equivalent of children-cum-teenagers which have been starved, and which are now by nature of their breed type and physiology going to be under demand to produce reproductively like adults, when they were never given enough to grow their bodies into adulthood in the first place. (It doesn't matter whether the eggs are fertile or not, creating eggs is a female's reproductive function and is a heavy demand on her system).
If you just get them layer feeds they will likely begin laying soon, but they've been ill done by, to be only fed corn and grits, so you can fairly safely bet on them dying of unnecessary problems not too long in the future if just put onto layer feeds. Given their previous diet, they are severely deficient in pretty much the whole spectrum of nutrients and to be a humane animal owner you need to give them a time to build up and heal from their deficient past before demanding your money's worth from them.
Layer feeds are developed to compel production from these types of hens, not just support it; for breeds such as Isabrowns and some other commercial layers production is not something their bodies will cease unless they're literally in dire straits. Normal hens of non-commercial breeds will only produce when in good enough condition but for commercial layers their condition is of secondary importance and it takes a serious problem for them to cease. Laying is not a sign of happiness as some think, nor of health, nor of a balanced diet.
Best wishes.
