Hi,

Maybe it's a bit to late, but I think I have a solution that can be helpful to other people.

You can change your display name by editing the launcher_profiles.json.

First of all, get access to the .minecraft folder (Windows + R, %appdata%\.minecraft).
Open the file launcher_profiles.json with notepad.exe.
Ctrl+f, type displayName and change your name after the dots ":".

That's it ! You will be able to change your ingame name by this way !