If you change your Minecraft username, you should fully log out of the game and then back in before playing again.
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@MojangSupport Still can't change my name, because you can't delete old minecraft accounts that haven't bought the game.
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Replying to @Sherrif_of_Nott
@Sherrif_of_Nott However, you can register those usernames in the Mojang account database. ^MW

Feb 5, 2015 · 11:00 AM UTC

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Replying to @MojangSupport
@MojangSupport you can't do anything with a free minecraft account, besides hold onto a name.