... If this ark is able to make a new planet, or even if it is limited to copying an existing one... Why not just do that to live on? Make a new planet (or copy earth, then modify the copy to suit their needs), store that, and then make another one whenever their population needs it. Why go through all this extra work with the bodyjacking? Unless this IS the "modifying the planet to suit their needs"... But this still seems like more effort than terraforming would be for their tech level.