May I ask, but isn’t that related to the “display name” under the Cloudflare → Account Home → Configurations → Preferences → Account Name, rather than the username itself?
Or it is related to an email which no longer exists or is not active due to some changes at domains, while it was, or it still is used to log-in to Cloudflare?
If you can log-in, you can either change the “display name” for your account and even your e-mail address.