[Radio Wrangler]

Cheapo crystals in plastic or soldered-together cases can get shifted due to muck getting onto the surface of the crystal.

Once we get away from contamination, qood quality quartz crystals have a logarithmic ageing characteristic. When they are made, their frequency is off of the frequency they will finally settle to. In their first week from manufacture they will drift so many parts per million in frequency. In the next ten weeks, they'll drift pretty much the same number of PPM. In the next hundred weeks, the same again, and then the next thousand weeks. and so on.

So if you order a crystal with tight specs from a reputable manufacturer, they make it, and then they keep it while it 'ages in' usually in a temperature-cycling oven to help it along.

So, your old crystals will be more stable rather than less

David