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.