Starting February 4th, you will be able to change a Minecraft username (this includes editing capitalization). If you sign in with your email, this feature will be available to you. If you sign in with your username, you'll need to migrate to a Mojang account. Usernames can be changed at
https://account.mojang.com/me just click the "change" link next to your name.
Does changing a username cost anything?
Nope.
Can I change to a Minecraft username that is already registered and paid?
No, that'd be crazy. You will, however, now be able to register usernames that are taken in other Mojang games, but are available in Minecraft.
How often can I change my username?
If you've changed your username already, you'll need to wait 30 days to be able to change it again.
What happens to my old username after I've changed?
If you change your username, your old name is held for 37 days, which gives you an extra week to change back if you've made a mistake or changed your mind.
If I ban someone on my server, and they change their username, do they stay banned?
Yes, and the same is true for whitelisted or OP'd players. Everything stays the same because the system is using your profile's unique ID instead of the username.
If I change my username, can someone see my previous usernames?
Yes. Username changes are set up so that you can get a username that you prefer, and are not designed to circumvent bans or anything of that sort.