If your organic feed recommends switching to Layer at 9 weeks, I’d seriously question their knowledge of chickens. In other words, they don’t know what they are talking about. I’m glad you were clever enough to ask. Layer typically has somewhere between 3.5% to 4.5% calcium in it. That’s for the hens to use to make their egg shells. But that much calcium can damage internal organs or cause skeleton problems with growing chicks. Do not feed Layer to growing chicks.
From your post though I don’t know that they actually recommend switching to Layer. I think you are just asking what to use.
What is the percent protein in that feed? A standard practice is to feed chicks a relatively high percent protein feed for the first month or two so they feather out faster and get off to a good start. That is typically in the18% to 20% protein range. Any time after 4 weeks and that bag of feed runs out, they switch to a lower protein feed, say in the 16% range. The idea, other than it probably being less expensive, is that it slows the chicks’ growth a bit, allowing the skeleton and internal organs maturity to better match growth. After about 13 weeks they switch to an even lower percent protein feed, a 15% Developer. This comes from the commercial egg laying industry where costs are very important. They also use the commercial hybrids which are more specialized than out normal breeds of chickens.
With our chickens, since we are not as specialized, we are not as restricted in what we feed. Plenty of people feed an 18% to 20% protein feed until they switch to Layer. Plenty drop down to a lower percent Grower. It’s not that critical. A lot of us never switch to Layer but instead offer an alternate calcium source, usually oyster shell, when they start laying and feed them anything from a 16% to 20% feed.
It may sound confusing but that’s only because so many different things can work. The only real rule is to not feed excess calcium to growing chicks.
Good luck!