It's the same as how you're currently leveling up, but with 2 minor distinctions:
1. The XP required to level up is much lower per individual level than it is in the late 40s 2. The first level of a specialization is free (e.g. you reached level 50 and chose to start working on "Operator". As soon as you chose Operator, you jumped to SR-51)
Is this finally a conundrum that CAN'T be solved by helicopter theft?