Minchi, saw you asking on the OT thread. In case you haven't read Fred's last post to you, it will probably make you feel better.
Everything I have read on other threads addressing this (including the OT thread) said go ahead and finish out the medicated starter you have and then change over to either the grower or layer, depending on what stage your birds are at. If you switch to grower and your girls start laying, all you have to do is finish out the starter and offer oyster shell on the side, and when you're through with the original grower, either continue grower with the oyster shell, or switch to layer at that point. All your birds are the same age (?) so they should all be at the same POL or close enough not to matter.
Just passing on what I have read. Lord knows I have no personal experience in this.
ETA: had to finish my thought about Fred's post