First, let a random Orc kill you. Then, play the resulting Vendetta mission and recruit the Orc (hold Circle to dominate him when his health is low). Go to the army menu, press Triangle on your new Captain, then press X on the current Overlord to make them switch positions and get the "Vertical Mobility" trophy. The earliest you can do this is after capturing the fortress in Núrnen. You need to be in control of the region's fortress to reposition the Captain. Also, you cannot recruit Orcs that are a higher level than your own. If the Orc is a higher level, write down his name, level up, and come back to recruit him later. Sometimes the reposition option is greyed out. If that happens, make someone else Overlord and try again, or issue the reposition command to your Overlord directly (instead of commanding the Captain).