I switched mine last year around the month mark. This year I had them on game starter (28%) only because I also am raising up turkeys. I switched them to 16 % chick starter at the two week mark.
There are some who will say leave them on the 20% and some that will switch them to lower as I did. Make sure they are getting the niacin they need/vitamins.
Dh book says at 2 weeks go to 15-16% protein. Too much protein too long may contribute to 'angel wing. You can add oats to diet too, he says it is good for them, like 10% or so.