A friend of mine was working on a map with me, when he decided he wanted to change his name. We have a scoreboard objective setup with his kill count, but when he changed his name, the scoreboard objective didn't respect the name change, and kept his old one. We're pretty sure that scoreboards use UUID's so we checked to see if when he killed another mob, his score would get updated. He killed a mob, and instead of adding to his old one, it added a new player to the scoreboard, and set it to one.

What I expected to happen was...:
His name would be updated in the scoreboard and scores would be added to that score

What actually happened was...:
His old name stayed, while his new name was added to the board, and started fresh.

Steps to reproduce
1.) Add a scoreboard objective /scoreboard objectives add Kills totalKillCount Kills. Set the scoreboard to the sidebar /scoreboard objectives setdisplay sidebar Kills
2.) Have a player kill a mob
3.) Change that players name
4.) Have them kill a mob again.
5.) A new player will be added along side the old one, even though they're the same player.