I don't switch when they are molting, I pretty much offer Layena crumbles in bowl, oyster shell in bowl, and scratch on ground. Only a little scratch for my young laying hens, enough to play with, and a lot of scratch for my roosters and my one older hen.
Symptoms - had a young hen have trouble with heat last year and try to die on me, then eyes closed in daytime and weak about 2 weeks ago, she is now in a dog crate by herself, ate scrambled eggs with shells yesterday, have given her a couple of calcium citrate pills one Saturday, one Sunday, but she's still kinda eating drinking and I don't want to overdose calcium. Her appetite for crumbles, or scratch, is not outstanding.
I've been keeping hens since 2010, and thin eggshells tend to turn up when the sand gets dirty so I refreshed sand in run yesterday. But this trying to die on me and needing a calcium pill daily has me concerned.