icloud username Help

I have changed my icloud username a while ago but on one device it just won't change it. iCloud does not now recognise the user name now as it no longer exists. I need to make my phone have the new one but its like its stuck in a loop and won't let me change it to the new primary user name. Has anyone else experienced this and have any answers to try and switch it to the correct icloud email address username?

Thanks