Later career's first terms aren't much worse than continuing in the same career:
New career, first term: one skill at 0, roll for survival. If you pass, roll for advance. If you succeed, roll for a skill at +1.
Continuing current career: roll for one skill at +1, roll for survival. If you pass, roll for advance. If you succeed, roll for a skill at +1.
I've figured it out. If a character got all service skills at zero every time they change career, the character could keep changing careers and get loads of skills at 0. But, as I said earlier, according to the rules, comparing a new career with continuing the current career, the difference is one skill at 0 compared with one skill at +1 respectively.
So, really, it's all about making character creation so that there is not such a big gap between continuing in the same career and having to start new careers.