That depends if you are planning on selling eggs or simply using them for your family.
If you are planning on selling eggs marked as organic, then you need to follow organic practices as approved by the FDA from the moment you get the chicks (2nd day after hatching, see cite below) throughout their laying career.
If it is simply eggs and meat for your family, you can feed them what you want when you want feeding regular feed now, organic later, and switching off and on as you like.
By switching to organic later, you may be circumventing some of the philosophy of organic feed...that the animal molecularly has never received "impure" or at least "non-organic" food sources. That is to ensure it has no undesirable elements, or contaminants, in its body so that it produces purer food products.
A female chick has all the ova she will ever have at the moment of hatch. As the hen comes to sexual maturity, every 26 hours, or so, an ova drops into the egg tract and over the course of 24 hours is made into an egg with the albumin (white) and egg shell being added to the yolk to form the complete egg that the hen lays. Then the process starts again.
Many small holders do feed regular food at the start and switch to organic just before point of lay.
My thoughts
LofMc
http://articles.extension.org/pages/69041/requirements-for-organic-poultry-production