Why Is Brompheniramine Maleate Prescribed Instead Of Pyrilamine Maleate?

My doctor has called a prescription in for me for the New Formula of Ru-Hist D. The OLD formula was: Pyrilamine maleate 30 mg & Phenylephrine HCI 30 mg. The NEW one is: Brompheniramine maleata 4 mg & Phenylephrine HCI at only 10 mg. WHY did the drug change? What different side effects are to be expected from the change in ingredient. Will the decongestive effect be diminished by having only 10 mg instead of 30? Thank You!