There is a lot being written and said about career currency. I come from the engineering standpoint (where I approach most matters!) and I find that there is not much prescriptive advice in the world to explain how to maintain or grow your career currency.
Definitions: Career currency is a person's total amount of knowledge and skills that can be applied productively in a field of employment or entrepreneurship. Career currency should be seen as synonymous with relevancy. Being an expert on the 1978 World Champion NY Yankees team does count as knowledge, but due to limited opportunities to apply the knowledge it likely does not qualify as career currency for most. Being an expert in seismic structural design in San Francisco (that is, building design for earthquakes) is a very potent career currency.
Dynamics: Currency depreciates over time, slowly. $10 today will be $10 tomorrow. But $10 today will not be $10 in 20 years. 20 years from now a Big Mac combo at McDonalds will cost $15-$20. And so it is with our careers, the skills that we acquire and develop early in our career loses value over time if left idle. To "maintain" your career currency you must add some every year, think of continuing education or periodic training. To "grow" your career currency you must invest.
An Academic View: Dr. Don Capener, Dean of the Davis College of Business at Jacksonville University does a good job dissecting the issue in his recent article here:https://www.linkedin.com/pulse/maintaining-your-currency-a-career-metaphor-dr-don-capener
Rules of Thumb:
4 years is make or break in a technical field. After 4 years "what you knew" has severely depreciated. In technology and advanced medical the timeline for depreciation can be much faster. Avoid depreciation.
Continuous education is the expectation. Utilize resources from your company or plug in with an industry association or group.
Investments (more below) have significant impacts in your career currency
Even in new roles, keep true to your roots by staying connected with the latest in Industry. If managing or administering a complex department, program, or project, find one portion of the technical aspect to "own" and stay grounded.
Invest: Investments in career currency can be seen as an investment in yourself. Like any investment, the greater the risk or the capital, the greater the expected reward. Graduate degrees, start-ups, career changes, non-profits, and moonlighting can all be forms of major investments. The theme is that these investments all have a common cost: your time.