X2. I feed Flock Raiser year round and offer oyster shell on the side. I just find it easier to not change feeds as I am usually hatching 3-4 times a year. Because of the heat that we are experiencing right now, I'm wetting the feed in pans for them rather than using the feeders so much (keeping feeders full though in case they want some dry). The difficulty with this method is that they tend to walk in the pans, squish (is that a word?) down the food, packing it into the bottom of the pan where they can't eat it. I don't give the cockerels any oyster shell, it doesn't seem to affect them not having it, but then again I've yet to get a cock bird to live beyond a year due to the neighborhood dogs. I should say though, that when they are in with the girls to breed, they can certainly have some of their oyster if they want it....I don't recall ever seeing them take any though.
I think what is best would depend on your overall management.
If your birds are confined and only get one type of food, then I would provide the oyster shells one week before I started switching out the diet (having both a small feeder or dish for the oyster shells, and throwing some on the ground daily in their favorite scratching areas to get them used to finding and consuming them), and I would wean them onto the new diet over a week, mixing about 10-15% more of the new food in every day. I would be sure to have the transition completed 1-2 weeks before the chicks were mixed in, if possible. In all likelihood they would be fine if you just switched them over one day, but this method gives them a chance to adjust, and avoids any temporary intestinal upset, calcium deficiencies, or food refusals because it tastes different.
On the other hand, if your birds regularly get different varieties of food, especially if they also free range, then they probably have iron guts and good food curiosity, so a lengthy transition would not be necessary. I free range my birds, throw oyster shells in their favorite scratching/foraging area daily, and mix 2-3 different brands of layer pellets together in their feeder at all times. When I change over I tend to just add a small feeder with only oyster shells (if I put them in a bowl, my silly Speckled Sussex rooster will immediately go through, i.e. empty, the entire bowl in one minute looking for something that he wants to eat, so at least the feeder keeps them in place for the hens), then mix my layer pellets and flock raiser in an "eyeballed" 2:1 ratio on day one, in a 1:2 ratio on day two, and then just flock raiser on day 3. I keep my feeders at the birds shoulder level, so I put pellets in the larger, higher feeders for the layer hens and crumbles in the lower, smaller feeders for the chicks. When the chicks are big enough to reach the higher feeders, then they are also big enough to eat pellets. (I really don't like crumbles -- too messy, which creates unsanitary conditions around the feeders within a few days, plus lots of waste, and my vacuum-cleaner piglets, aka layer hens, tend to choke on the crumbles, then go to drink water with the crumbles around their beaks and it gets in the water bowls which fouls the water, so I try to get my birds on pellets asap.)
My hens are free range 9am-9pm most days so I think I will do the switch over just a few days, as they are eating much less pelleted feed since there's an abundance of yummier green stuff in their pasture.
I thought about creating a creep feeder (we used to do this with other baby farm animals on the farm I grew up on) where the chicks feed is in a place only they can access, and the grown hens feed is up high enough the chicks can't get to- but I don't have much experience with chicks so I didn't know if it would work...
I have always had oyster shell and grit in the coop free choice for them, but I have trouble with one of my golden comets who has maybe missed 3 or 4 days in laying since she started in the fall- her eggs are huge and if I don't give her extra calcium mixed in a treat 1-2 times a week her shells get thin, to the point where they break when touched

Thank you both for sharing how you deal with your chicks, I greatly appreciate it
