According to the Final Career section of AG:9, a PC forevermore gains skills as an expert in their Final Career, whatever that is. I have always had a problem with that. Consider for a moment a pair of PCs from frontier worlds (one of whom begins as an Academic, and then becomes a Troubleshooter, the other beginning as a Scout and becoming a Contact)...
Supposing that they both roll a base of 14 for Education, 13 for Intelligence, and 15 for Determination, they will have 14 + 2 + 4 for a final total of 20 in Education, and receive 10 background points. Just to make things easy, suppose that they both put one point each into Combat Rifleman, Sidearms, Melee, Ground Vehicles, First Aid, Survival, Electronics, Mechanics, Swimming, and Vacc Suit.
Next, the first one becomes an Academic majoring in Biology, with minors in Chemistry and Psychology, putting a few points into Writing and the Intellectual skills available, as well. After passing one or more turning points, they become a Troubleshooter, remaining one for the rest of their life. The second PC spends two terms in Scouts, putting points into all the Primary Space Crew, and as many Related Military skills as they can afford, before becoming a Contact team member. How does this affect their skills development for the rest of the PCs' lives?
Troubleshooters have Primary skills of: All Combat, General, Underworld, and Vehicle skills, and Information Gathering. Related skills include: All Space Crew skills, Bureaucracy, Computer, and Psychology. Contact team members have Primary skills of: All Academic skills and Ground, Hover, and Sea vehicles (Sidearms, Survival, and Vacc Suit are included due to the Increasing Skills section (AG:10)). Related skills include: All General, Intellectual, and Journalistic skills. Since the Background Skills section (AG:9) says that "Referees may permit players to spend career skill points on background skills." If this is done, it basically raises them to the level of Primary skills.
So, if the first PC had taken at least one level of Biology, Chemistry, Psychology, all the Intellectual skills, and Writing, then the Intellectual skills and Biology would be Primary skills (for as long as they remained in that career), and Chemistry, Psychology, and Writing would be related. Upon becoming a Troubleshooter, however, Biology drops from a Primary to an Unrelated skill, Information Gathering remains Primary but Bureaucracy and Computer drop from Primary to Related, Chemistry and Writing drop from Related to Unrelated, and Psychology remains Related.
For the second PC, if they had taken all of the Primary Space Crew skills, most of the Related Military, and Air Vehicles skills, then switched to Contact, all of these skills would then become Unrelated, forevermore!
Why? Why would a PC's ability to learn more about their old college or job skills suddenly take such a dive? In the first case, all of their background skills are included as Primary skills in their Final Career, so they all remain Primary, but if their Final Career had been Contact, most of them would have dropped to Related, instead (with the exceptions of Sidearms and Ground Vehicles). Needless to say, I maintain that it wouldn't work this way.
The following suggested set of rules make the skills selected during character generation far more important to a PC's development, but requires no more record keeping than the usual method. This makes every PC even more unique. Basically, it assumes that skills, once gained, remain at least as easy to improve as they were to gain (they may even get easier).
As an optional rule, the Referee may or may not want to include a rule for allowing players to customize their PC's Skills Set: Any Unrelated skill which the PC raises to Skill Level 1 during character generation becomes a Related skill for that PC. Any skill raised to Level 2 becomes a Primary skill. Thus, if PC #2 above (who has no access to Underworld skills) had developed Stealth 2, (s)he would thereafter have it as a Primary skill, instead of an Unrelated one. Referees will have to decide on what level of development is required for gaining Primary/Related status, and whether or not to even allow this.
Thus, PC #1 has Primary skills of: All Combat, General, Underworld, and Vehicle skills, Biology and Information Gathering. Related skills include: All Intellectual and Space Crew skills, Chemistry, Psychology, and Writing. All other skills are Unrelated.
PC #2 has Primary Skills of: All Academic and Space Crew skills, Combat Rifleman, Electronics, First Aid, Ground and Hover Vehicles, Mechanics, Melee, Sea Vehicles, Sidearms, (optionally Stealth), Survival, Swimming, and Vacc Suit. Related skills include: All General, Intellectual, and Mercantile skills, Air Vehicles, Archery, Combat Walkers, Demolitions, Forward Observing, Heavy Weapons, Leadership, Recconnaissance, Tactics, and Thrown Weapons. All other skills are Unrelated.
As you can see here, these two PCs have unique skills sets, based on their players' decisions during character generation. One thing you will begin to see happening is that Ultra-high skill levels will tend to become more uncommon, as the players attempt to get their favorite skills as Primary skills, instead of pumping skill points into just one skill. In my experience, this is a good thing. It allows for a broader skills base among the PCs, and allows a player to generate the type of character they want to play more easily. The only other piece of record keeping needed is: which skills are Primary and Related. That can be recorded on the Character Sheet with a pair of differently colored pens, putting a red line in front of Primary skills, and a blue one in front of Related, for example.