If you don't mind using mods, there is one called "Change Career Branch" that does exactly that. I've never used it, but it seems to still be working flawlessly.
If you don't want to mess with mods, there used to be another way. I'm not sure if this still works, but exporting a sim to the gallery should retain only the current job, forgetting the job history. So, you could:
- Take everything out of that sim's inventory (as the inventory isn't retained).
- Change the sim's career to a different one (doesn't matter which, as long as it's unrelated to the one you want to change the branch).
- Export that sim.
- Start a new game with that sim.
- Have the sim rejoin the career you want to change the branch.
- Export the sim again, and merge him into the original save he came from.
Keep in mind that the career branch won't be the only thing missing. And, please, backup your save before you try it; I don't want to be held responsible for someone messing their game.