How do I switch from using a -5 bat to using a -3 bat?

Im fifteen years old I weigh 160 pounds but am currently doing P90x so ill probably be about 140, I am five foot eight and my coach reccomended that I buy a -5 bat that is 28 ounces 33 inches, so I bought an Easton reflex. I wanna use a heavier bat so I have more power and wanna buy an Easton Omen or Surge, so will working out my upper body in the weight room more instead of running help?