That question is a lot harder for me than it should be, but there is an easy solution. The solution is get another bag of whatever you are feeding them and offer oyster shell on the side. Other than protein level, the only significant difference is probably the percent calcium. The oyster shell will solve the calcium problem. I don't see a huge problem with protein levels, as long as you don't get carried away.
If you are interested, I'll tell you whyt that is a harder question than it should be. There are plenty of studies out there where they clearly prove that feeding Layer to growing chicks can cause problems. There is a difference in "can" and "will", but they cut the chicks open to see what that extra calcium does to their internal organs. They also count how many chicks actually die. There is no question that feeding Layer is bad for very young chicks.
The problem is that those studies all start feeding Layer at hatch. I'm not aware of any relevent studies about starting Layer at 2 weeks, 4 weeks, 8 weeks, any of that. I don't know at what age the extra calcium ceases to be a problem. There is also the issue that the percent calcium is not really that important if they eat a lot of stuff that is not Layer. The problem is in how much total calcium they eat in a day. And it is not even a day problem. One bite won't kill them. It's how much total calcium they eat over time. To me the answer is not really clear cut.
I think yours would probably be OK switching to Layer at 15 weeks. I think, but I don't know. And with such an easy solution, offering oyster shell on the side, why take chances?
