I'll get into the "can it be changed" question later - having some conversations off line with Paul. However as long as the crown gasket has not turned to black goop, you can use it on the watch without any problems - no water resistance of course but I do this for my customers all the time.
If the crown gasket has turned to black goop - it gets to be like tar - then be aware that over time I've seen that tar like substance travel into the movement on some watches. So if the gasket has gone that way, I would either not use the crown, or take the steps to remove the black goop. I've found the most effective solvent for this goop is alcohol, so I use 99% at the bench for this. But the gasket will also burn, so that is another way of getting it out of the tight space in the crown. After burning it out (in a place where the smoke won't cause you problems) you can then run the crown through an ultrasonic cleaner to remove the debris.
Cheers, Al
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