If you mean you have used the change username option for a VM email address then the new username has to be a third party email address that you already have access to. It cannot be another Virgin media email address. Changing the username does not change the underlying email address.
You can cancel the process by signing back into My Virgin Media with the original username and password.
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GrahamI'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media, I'm a VM customer. There are no guarantees that my advice will work. Learn more
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