Changing to stage 2 formula

My DS is six and 1/2 months old, and is on stage one heinz nurture but has had trouble with constipation so am concerned about changing to stage 2 which is casein dominant, have been told that casein can make constipation worse due to decreasing the transit time through the body anyway, it does however have increased levels of iron etc so don't want to deprive DS of those nutrients, but obviously don't want to make constipation worse, has anyone kept there child on stage one until one year, is this ok ,he will soon be on three solid feeds per day so will be getting added nutrients from that anyway.