I was thinking about this for some time now.Can ghosts become spirits or spirit-like-ghost-things?
I think if something like this would happen it would be either:
Passively : a ghost stays too long in a place where a compatible spirit would be borne
Actively : a ghost go out of their way to add more spectral energy and characteristics to themselves,the only way I can think of this working is eating spirits and other ghosts, but i don't know if ghosts can eat in that sense of the word.