what is being changed is exactly what is says - the username. This is the 'handle' or 'nickname' (VM call it a username) by which you log into the the email account in webmail.
Usually the email address itself is the username for that account, so the two can seem to be one and the same thing since they both share the same appelation. But they are in fact two different things, one is an email address, the other is a username for logging into the email account.
Maybe think of your bank account, the bank account has a username for logging into it, that username might be the account number itself, or it might be an email address of your choosing. But changing this username does not change your bank account number.
Just as with changing the username on your bank account (or most other online accounts), a confirmation is sent to your chosen email address for you to confirm that you own the email address. The email address you choose must already exist, it is not created by the fact that you choose it as a username. So quite what happens if you mistakenly choose an email addresss which does not exist in the belief that you were creating a new email address I do not know. But I guess that the lack of any activation back from your fictitious address will mean that the status quo prevails.
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