"You didn't change much."
Translation:Tú no cambiaste mucho.
He didn't change what? Himself? Should it be reflexive "tú no TE cambiaste" here?
If he didn’t change many items on the new meeting’s agenda, then Duo’s sentence is 100% correct. If you are speaking of a person changing his/her self, use the reflexive pronoun. Without context these sentences can be maddening. Just go with the flow and try to learn as much free Spanish as you can.