How do I transfer the contents of a player's enderchest to a different account?
All ender chest items are stored in each player's unique playerdata files that can be found in "World\playerdata". Just like how you transferred the "Inventory" tag to the new player to copy their inventory, you can also transfer the "EnderItems" tag to the new player to copy their ender chest items. Just make sure the server is comepletely closed before you do this.