replacing vs substitution

replacing and substitution both are nouns.

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

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

replacing: the act of furnishing an equivalent person or thing in the place of another
substitution: the act of putting one thing or person in the place of another:

Other hypernyms of replacing include commutation, exchange.

replacing (noun)	substitution (noun)
the act of furnishing an equivalent person or thing in the place of another	the act of putting one thing or person in the place of another:
	an event in which one thing is substituted for another