afterthought vs change of mind

afterthought and change of mind both are nouns.

Word	Noun	Adjective	Verb	Adverb
afterthought	Yes	No	No	No
change of mind	Yes	No	No	No

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

Other hypernyms of afterthought include flip-flop, reversal, turnabout, turnaround.

afterthought (noun)	change of mind (noun)
an addition that was not included in the original plan	a decision to reverse an earlier decision
thinking again about a choice previously made