Paternity question in PA

If a divorce decree is finalized and there is an order for child support and then the paternity of the child comes into question can the alleged father circle back around and compell a paternity test?

I have been told by some that it is easy. Others tell me once you make that first child support payment you have admitted to paternity and unless you can prove the mother flat out lied you are on the hook for the support until the child is 18/graduates period.