The organic rules state that once an animal, chicken or anything else, has eaten something not certified as organic the animal can never be considered certified organic. So as far as certified organic they are forever excluded. But I look at it as organic is a way of life, not a set of rigid rules. If you want to switch to organic feed there is nothing stopping you, you cannot claim them to be organic though.
You will get some differences of opinion as to when to switch Layer. Commercial operations switch at 18 weeks but they pretty much control when their chickens will start laying by manipulating light and feed. They make the days longer and increase the amount of protein in the feed to start laying. It's generally only a 1% change in protein levels, going from 15% to 16% protein. Genetics plays a big role in that also, theirs are bred to start laying at a certain age. Our chickens are not. That 18 week recommendation is based on the commercial flocks, not our backyard flocks. Still, a lot of people use it for backyard flocks and do OK.
The studies I've seen show that excess calcium is bad for growing chicks. I'm not sure how much excess calcium harms chickens that have pretty much stopped growing. Some people think it is a lot but others don't worry about it and generally do OK. To me starting them on Layer when they start to lay is not a horrible thing but I don't do it. I almost always have growing chicks in my flock so I feed everyone the same low-calcium feed and offer oyster shell on the side. That way the ones that need it for egg shells eat it while the others don't eat enough to harm themselves.
If you look at the label on a bag of feed you should see the analysis. This shows you the percent of certain essential components of feed in that bag: protein, calcium, fats, fiber, salt, and a few other things. In the labels I've seen all the components are pretty close to the same except calcium and protein. There are some variations but they are pretty minor. As far as I'm concerned you can fee your flock any of them as long as you pay attention to calcium and protein. But check the labels, especially if you go organic you may be using a feed I'm not familiar with.
