L.B.
I am having the constipation problem currently with my 9 week old. I'm told by the pediatricians that switching formula is a last resort. I have been giving our son 2oz. of apple juice 2x/day and it seems to be working pretty well. The doctor said if apple doesn't work, we would go to prune juice. It was a bit surprising to me that they don't change the formula unless there are allergies or unless the problems are extreme. My daughter is 15 and I recall changing her from Similac with Iron to Similac without or low iron.
If you haven't already, I would check with the ped. before going any further.