Start a new game with the exported household, but only keep the sim you wish to change the career of. Save the game and go back to the main menu. On Save A, have the sim you want to change branches leave their job, join a different one, empty that sim's inventory, then export the household to the Library. You don't want to mess your existing save if this fails. Use 'save as' to create a new copy of your save.
In theory, if you want to avoid mods, you can: 1.
