Office 365 Cutover Migration Error

I'm doing a Office 365 Cutover migration and get a error for 1 user. Office 365 was setup with the default user named admin@lufber.nl. But the mailbox that I want to migrate from the existing exchange server had also the name admin@lufber.nl. So I get the  error: ProvisioningFailedException: The name "admin" is already being used. Please try another name.

I have tried to change the username in Office 365 for this default account that already exists but that didn't work. And when I delete this account then the migration is not working anymore and get the message "failed" for all mailboxes. It look likes that Office365 is using the default account or something for the cutover migration.

So is there a workaround that I can use so that I can migrate the admin@lufber.nl account?
Emiel ZwartEngineerAsked:
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Vasil Michev (MVP)Commented:
The cutover process uses the WindowsEmailAddress (mail) attribute for provisioning new accounts. In theory, you can change the value for that attribute for the on-prem mailbox, and the migration should be successful. But this will of course provision a new account in O365, which might not be what you are after.

Perhaps a better solution is to change the WindowsEmailAddress of the cloud account (using Set-Mailbox -WindowsEmailAddress new@domain.com) and any other occurrences of the same address in relevant attributes (email addresses, external address, etc).
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