It does recommend switching at 18 weeks - I did! I was very sorry for doing that because my girls are delayed in laying and it could possibly also be the fact they were not getting what they should have had from grower feed.
CAL
Starter feed - if you get the medicated one is medicated - I don't believe grower feed is medicated - It has more protein and Lysine and the other methionine
CAL
I would not be in a hurry to switch feed until they actually start to lay. Compare the ingredients and the percentages first Not just the protein but the Lysine and methionine. I would clear away any blockages to the nest boxes but be prepared for them to poop in them.
My Barred Rock layed...