How do I change my Minecraft "displayName"?
I found a workaround for this!!! You have to use the old version of the launcher for this to work again. To do this, go to the minecraft launcher and right click to go to properties, then go to compatability mode and use Windows XP(service pack 2). Once you set this open the launcher. It will give you a message asking if you want to keep the compatibility settings. Choose yes, (its the option on the right for me). Close the launcher, edit the launcher_profiles.json and open the launcher again. It worked for me, even with new versions. Hope it helps you too :)