in certain situation, when the key cannot be polished, then the key cap can be replaced.
common thing happen to the key is the bushing. if the bushing (balancing bushing and front key bushing) are not sticked properly, they will have full contact with the key and balancing pins, that will cause friction. so sometime when you press certain keys, you will find that the response are slow or the key will not fudge, well this is one of the reason that cause.
the other is common problem is the silver fish a type of bug which like to eat the bushing felts and clothes. when the bushing are all eaten up, it will cause the piano keys wobble and shake...it will affect the piano touch.
please enjoy the short slide i have created