There may be a cheat to promote a Sim to a specific career level. I wasn't aware that you couldn't demote down to before the split without a sim getting fired first. What happens when they try to rejoin the career after being fired from it? Do they start on the same level as well?
I'm fairly certain there isn't a way to just switch a branch without using a mod. If I'm remembering correctly MCCC will allow you to basically demote down to before the career split and you can re-choose it that way. At least it used to allow that I'm not sure if it does anymore. There also
THIS standalone mod however it hasn't been updated since 2015 so I don't know if it works any longer.
I agree it's really annoying that you can't have one sims do all the careers without the use of mods/a lot of monkey work. This definitely needs to be changed but given that it's been that way since the game's launch I doubt it's going to get changed at this point.