Question regarding older phono preamp

Semiconductors don't degrade unless they are abused.  Neither do resistors or transformers; again, unless they are abused.   In your case that would mean storing it in an attic at a high temperature (think summer) or super cold basement in winter where it might also be damp. 

Capacitors, switches, and relays however are a different story.  If they are sealed, then they will last a very long time.  If they are not, then they will degrade from oxidation. Relays and switches also have a lifetime based on the number of operations they perform.  For a preamp, the input selector switch gets all the use, so if it works okay, you should be fine.  When relays go bad they tend to stick, so they are working okay, you should be fine. 

Case in point, I have a number of preamps, one of them was built in 1977 and is still going strong.  I had a Dynaco PAS 3 preamp, tube, although I sold it some years back, it was working just fine and it was 100% original factory.  It still sounded great and I am sure it dated back to the early 1960's.

Seriously speaking, if the unit is working okay, there is no need to upgrade.  Now if you want more audio performance, then a good modern preamp should be a significant upgrade sound wise.

@spatialking Thanks for the detailed response! Im definitely looking for more audio performance.  An upgrade is on the horizon after reading your post. 🙂

Electrolytic capacitors fail over time. No matter what. But lack of use is worst case. if your unit was in storage for more than 10 years, you might have someone check the power supply caps. Film capacitors do last essentially forever, unless they have seen a voltage that exceeds their rated maximum voltage. In general, if your unit sounds OK, the capacitors are probably OK. Among resistors, carbon composition resistors (brown cylinders with colored banding) change value over time. In certain cases, this can throw the circuit out of whack and cause poor sound quality or even damage other components. They rarely fail completely, but they do change value over time.

Usage is what causes issues.  Time and heat. Since we have been repairing and upgrading components for a very long time, the main issue  is heat and chassis design. Many chassis look so beautiful but when you open them up, the parts are squeezed in side trapping heat.  Boards get brittle and solder traces can go bad.  Capacitors eventually also go bad.  If you like what you have, find a good repair tech and upgrade the parts and then don't look back.  A new unit will not contain better parts so upgrade and enjoy.