constancy vs unchangeability

constancy and unchangeability both are nouns.

Word	Noun	Adjective	Verb	Adverb
constancy	Yes	No	No	No
unchangeability	Yes	No	No	No

As nouns, unchangeability is a hypernym of constancy; that is, unchangeability is a word with a broader meaning than constancy:

constancy: the quality of being enduring and free from change or variation
unchangeability: the quality of being unchangeable; having a marked tendency to remain unchanged

Other hypernyms of constancy include changelessness, unchangeableness, unchangingness.

constancy (noun)	unchangeability (noun)
the quality of being enduring and free from change or variation	the quality of being unchangeable; having a marked tendency to remain unchanged
faithfulness and dependability in personal attachments (especially sexual fidelity)
(psychology) the tendency for perceived objects to give rise to very similar perceptual experiences in spite of wide variations in the conditions of observation