improvement vs change of state

improvement and change of state both are nouns.

Word	Noun	Adjective	Verb	Adverb
improvement	Yes	No	No	No
change of state	Yes	No	No	No

As nouns, change of state is a hypernym of improvement; that is, change of state is a word with a broader meaning than improvement:

improvement: the act of improving something
change of state: the act of changing something into something different in essential characteristics

improvement (noun)	change of state (noun)
the act of improving something	the act of changing something into something different in essential characteristics
a change for the better; progress in development
a condition superior to an earlier condition