It is true that Napoli has changed their poor performance, and they were lucky enough to win the Scudetto this season, but I don't see Napoli as the favorite to win the Scudetto next season because if not for Inter Milan's busy schedule in the UEFA Champions League competition, Napoli wouldn't have won the Scudetto this season. In my opinion, Inter Milan is the favorite to win next season's Scudetto because they will want to make up for their mess next season.
It's still uncertain who will be the favorite for next season, given all the improvements each candidate has made. It's unclear whether they'll improve or decline, honestly though, Napoli looks more promising to me because Conte is still in charge, so he just needs to address last season shortcomings. Inter shouldn't be underestimated but under a new coach they'll certainly have some adjustments to make especially given Chivu lack of experience. For more clarity we'll have to watch their last four or five Serie A matches to get a better idea.