supersession vs replacing

supersession and replacing both are nouns.

Word	Noun	Adjective	Verb	Adverb
supersession	Yes	No	No	No
replacing	Yes	No	No	No

As nouns, replacing is a hypernym of supersession; that is, replacing is a word with a broader meaning than supersession:

supersession: act of replacing one person or thing by another especially one held to be superior
replacing: the act of furnishing an equivalent person or thing in the place of another

Other hypernyms of supersession include replacement.

supersession (noun)	replacing (noun)
act of replacing one person or thing by another especially one held to be superior	the act of furnishing an equivalent person or thing in the place of another