Minecraft Singleplayer Whitelist?

I have the following problem, I changed my player name on the website and can't switch game modes anymore (from survival to creative). As it stands then, that I do not have the permission for it. How can I change that again?

So when I changed my player name, I first had to log out of the minecraft launcher and log in again. That should actually work for you.

This happens in a singleplayer world? Strange.
You can open it on ESC → In Lan and allow it on cheats: GO. Then you can cheat again. However, just solagne, bite you again go out of the world. If you go back in you have to do it again.