If you quit a career just after you had to make a branch choice, you will restart exactly at that point.
For example:
Tech Guru career level 7. You choose e-Sport Gamer. Your job title is now: e-Sports Competitor.
You quit and take a new career. Anytime you want to go back to the Tech Guru career, you will start again as a e-Sports Competitor
Changing careers at the right time is important. You do not want to have to take certain levels twice.