"Impersonating" won't take place as your old username is saved in reserve for 37 days, meaning that most people you know would already know of any changes made, people who try and say they're you will just be proved wrong.
You have to go to your Mojang account settings and then to your Minecraft account, assuming it would be a pretty visible option, there would also probably be an announcement of sorts about how to do it.
Comments on Profile Post by GizzBots