If you have both P5 figures and P2 figures available just overlap the positioning so you can see both figures and start scaling the P2 figure so that shoulders, elbows, wrists, waist ,thigh,knee and ankle joints match up. Then save the new scaled P2 figure. It may look grotesque but should work ok. However since BVH stores the joint positioning by angle you need to hope that the joints bend in the same direction. You can test this by applying bends to each joint in order and check they match.