I was hesitant to answer this question because I don't really know what I'm seeing. I have not noticed any of the few greys I do have changing but who knows, in time after the old greys fall out perhaps new colored ones will grow in it's place. It is interesting to question what it means if they do not change. I still do not understand why a child can have dark hair when born, then very light, dark again and grey in old age. I know of someone like this, even though it may not be the norm.
Many children, may have light hair when young and then it gets darker as they get older and then goes grey as an older adult.
Turning grey is not the only change. It also gets coarser.