We're lucky to have a career in such a dynamic industry. The technology just keeps progressing year after year. Change is so fast that it's often a struggle to keep up. In the rush to learn the latest new thing (which you probably won't use in your job for a while) you must also keep up to date with your old skills.
Use it or lose it
There's an old saying "use it or lose it". This is so true and applies to all skills. About 5 years ago I was doing a fair bit of SSRS (SQL Server Reporting Services) for clients. It was pretty bad and unreliable at the time with a lot of crashes and unexpected behaviour. It got to the point where I tried to avoid SSRS because it was so crap.
Fast forward five years and I have suddenly been called upon to be the expert of SSRS, mainly because no one else in the entire company has ever used it! With trepidation I installed SSDT (SQL Server Data Tools ) for Visual Studio 2015 and then racked my brains on where to start. After a few minutes it all started coming back. This time I was targetting SQL2014 which seems to be infinitely more reliable. Not much seems to have changed. The same controls and charts were still there. The report server interface still sucked and only really worked in IE.
After an hour or so I was back up to full speed, smashing out reports as per spec which would be earning the company big bucks $$$$. It made a nice change to my normal software development day. It made us money and I refreshed some old skills that are still in big demand.
You may think that some of your knowledge from years past are not worth keeping fresh, but in the world of tech the old is still in demand as the new. If you already know an older skill, try to keep it fresh as you will never know when you need it or use it to your advantage.