How to can change your minecraft user name ofline?

Here is what I found that worked. On Windows 7 and Vista. After having ran the launcher once to install the new update:
1. Click the Start button and type "Run". Select the run program.
2. Type %appdata% in the run dialog.
3. In the ".minecraft" folder, look for a file named "launcher_profiles.json". Open the file with notepad.
4. In the file will be a string of text. In the section "displayName": "YourUsername", change your username to whatever you want.
5. Save and close the file and run the launcher again.

This worked for my son & I. We were able to play over LAN after doing this.

Hope this helps!