The User is still inPrem, but has already a (empty) Mailbox in Exchange Online.
To delete the Mailbox in Exchange Online, run this:
Get-User [UPN] | Select name,*Recipient*
Name PreviousRecipientTypeDetails RecipientType RecipientTypeDetails
---- ---------------------------- ------------- --------------------
00ea000a-0d00-0000-bd00-00b0000aae0d UserMailbox MailUser MailUser
Set-User [UPN] -PermanentlyClearPreviousMailboxInfo
Get-User [UPN] | Select name,*Recipient*
Name PreviousRecipientTypeDetails RecipientType RecipientTypeDetails
---- ---------------------------- ------------- --------------------
00ea000a-0d00-0000-bd00-00b0000aae0d None MailUser MailUser«PreviousRecipientTypeDetails» should be «None» after this. Now you can assign a Exchange Online Lizense to the User.