Arkathan said:
This should be very limited, and the loss of skill should occur prior to beginning training for the new skill.
It should be reserved for skills they haven't ever used, and preferably taking an earlier skill over a later one if several have never been used.
It's hard to forget something you used at a proficient level during the time span of game play, and really hard to go below level Zero. I've forgotten a lot of what I did in the military, but if presented with materials, can still go through them with some level of familiarity more than thirty years removed. So an earlier career is also a good candidate for that pool.
Depends a lot on the type of learning. For instance, the kinesthetic memory you develop doing immediate action drills may have more traction than something you read in a university course and then didn't need other than for the exam.
I know when I came out of school, I probably had:
Math-2 (if you don't want to break it out to cover descriptive & inferential statistics, goodness of fit, basic calculus, basic integrals, differential equations, partial differential equations, laplace and fourier transforms, queue theory, ring theory, graph theory, NP-completeness problems, numerical methods, algebra, trigonometry, tensor calculus, network theory, matrices for fun and profit, n-dimensional math, and enough other kinds I forget - I trained in continuous math on the way towards an engineering degree and then in discrete math in computer science)
Rifle - 0 (reservist)
Computers - 3
Recon - 0
Stealth - 1
Communications - 0
Ground Vehicle - 1 (a bit of rally racing and a bit of special survival driving)
Geography - 1
Geology - 1
Chemistry - 1
Physics - 1
History - 1
Mechanics - 1
Electronics - 2
Steward - 0
Russian - 1 (or Linguistics - 1)
French - 1 (or maybe still Linguistics - 1)
Handgun - 0 (rec shooting)
Martial Arts - 2 (used to train 6-10 hours a week in Aikido and prior to that Karate with a bit of Aiki-Jutsu thrown in)
Medic - 0 (military and civilian first aid)
Sport -1 (raquetball, squash, hockey, softball)
Over time, I'd say now:
Medic - 1 (experience and more training)
Steward - 1 (training and lots of experience)
Russian - 0
French - 0 (or 1/2)
Physics - 0
Chemistry - 0
Geography - 2
Recon - 1
Stealth - 0 (due to debilitating injury)
Martial Arts - 0 (same)
Sport - 0 (knowledge still 1, ability to execute 0)
Mechanics - 1
History - 2
Geology - 1
Ground Vehicle - 0 (eyesight and injury prevents high speed manouvering)
Communications - 1
Computers - 3 (maybe 4... I've done a lot in many different domains and some quite complex and large scale)
Intimidation - 1
Political Science - 1
Admin - 1
Math - 1
Tactics - 1 (training, simulations, and knowing lots of vets and quite a few SF as close friends)
Electronics - 1
Instruction - 1 (taught college course, tutor kids)
Leader - 1 (led team, had training)
Carpentry/Construction - 1 (done a lot of work and worked with contractors)
By my count, that's 22 skills whose levels will have changed, comparing me getting out of school at 26 vs. me now at 52.
My list of skills has changed a lot. New ones added, other suffered significant downgrades due to (in Traveller sense) aging crises which made some physical skills MUCH less executable. Also, if you don't keep up the details of skills like Electronics or Martial Arts, you don't forget the basics, but there are minor subtleties which really magnify your impact and results and these tend to be what you forget and don't easily just reclaim. Meanwhile, I've read broadly in many areas and become very politically aware and educated.