Changes
-The battle against Astrid lasts longer. Ash's interest in Mega Evolution grows.
-Fennekin hears Diancie instead of Pikachu and Serena protects her instead of Ash.
-Bigger connection between Diancie and Serena than with Ash and Diancie (basically switch Ash and Serena in their scenes).
-Serena tells Diancie about shooting stars rather then Ash. We hear Diancie's wish that she could be with the group forever. (or for longer)
-Serena catches the Mega Diamond instead of Bonnie which Ash recognizes as a Mega stone. He tries to mega evolve Diancie but fails.
-Serena catches Diancie at the mall and Ash sends Fletchinder to catch them.
-Serena gives Diancie the confidence to Mega Evolve and uses the Diamond from earlier to Mega Evolve her.
-Serena holds onto the Mega Stone and Diancie silently promises to return to her one day

Additions
-More focus on relationship between group
-More scenes depicting the journey to Allearth Forest. (also cut out return to Diamond Domain. You didnt get the fairy Aura, why would you come back?)
-Larger action scene between Mega Diancie and Yveltal. Xerneas deals the final blow with a Moonblast.
-More background and interaction between Ninja Riot, Marilyn, and the Steels