On the Mojang Support page it says that if you change your username the old one will stay locked for 37 days. You can change your username 30 days after you last changed it. This gives you a week to decide whether or not you want to go back to your old username.
Does this only apply the first time you change your username, or every time that you change? Also, does it only keep the username you're changing from, or the initial account name?