Tracy,
You may get lots of different advice, but here's what I would do if I were you. I would continue with the chick feed until they are at least old enough for egg laying. It is a little difficult (read impossible) to have the feed sack end at the exact time that you need to switch feed types, but that needn't be a problem.
It is important to have your chickens get everything that they need while they are still developing. You didn't say what breeds you have, but in general the earliest that chickens would lay is 16 weeks or 4-months. Some breeds wait until 7-months old to start. So those chick feeds with 18-20% protein will probably help guarantee healthy chickens. Once they start to lay you can switch to a layer feed, or you can get 'flock feeder' feed which is generally higher in protein than layer feed and always have oyster shell or other source of calcium available to them.
There is no harm what so ever in making oyster shell available to your chickens all the time. If they need it they will eat it, if they don't need it they will ignore it. I have a cage cup of oyster shell and one of grit there so that they have what they need should they ever require either of those.
Lots of nice fresh water and adequate food 'free choice' are the approaches most people take. Chickens know what they want/need, so oyster shell available even if you chickens are beginning to lay could easily help you finish off your bags of chck feed in the event that you have early layers.
Good luck with what you decide.
