I've been in three different unions before I landed in this career. Salary increases were fixed by time and position. Position was always based on seniority. No exceptions. You could try to transfer locations within the same company to try and get better roles (which is how most people got promotions, aside from waiting for people to die).

If you changed companies though you started at the bottom, guaranteed, because if you get moved ahead of people in the new company it creates an utter shitstorm. Management at the new job basically has to hate their employees and themselves to do such a thing.