fixity vs changelessness

fixity and changelessness both are nouns.

Word	Noun	Adjective	Verb	Adverb
fixity	Yes	No	No	No
changelessness	Yes	No	No	No

As nouns, changelessness is a hypernym of fixity; that is, changelessness is a word with a broader meaning than fixity:

fixity: the quality of being incapable of mutation
changelessness: the quality of being unchangeable; having a marked tendency to remain unchanged

Other hypernyms of fixity include unchangeability, unchangeableness, unchangingness.

fixity (noun)	changelessness (noun)
the quality of being fixed in place as by some firm attachment	the quality of being unchangeable; having a marked tendency to remain unchanged
the quality of being incapable of mutation	the property of remaining unchanged