Since the active element is a strip of Aluminium, a very inert metal due to its very strong, nonreactive oxide layer, I would say ribbon mics probably change less over time compared to dynamics or even capacitor mics?
It is often said that magnets loose their power over time and that would affect sound quality, and of course sensitivity. The fact is it ain't true! Magnets do not fade, not unless they are subjected to physical shock. Every time you 'bang' a magnet you jostle the magnetic domains and some settle back to a more random orientation. That is lost of flux. It is unlikely that something so 'precious' as an expensive ribbon mic would be subjected to such treatment. One drop would have little effect (but is very likely to knock the ribbon out of alignment!)
The biggest enemy of ribbon mics is dust clogging the flux gap and a tiny fraction of that dust is magnetic (known as "tramp iron") and can 'short' the magnetic circuit. Always said to be best to leave the mics on a stand, keeping the ribbon vertical covered with a poly bag.
Dave.
