Self-service users and AAD Connect

Moe_Kinani Thanks for your reply.

I tried your proposed solution, with the following results:

1. On my colleague's account, the one who registered himself to get access to Teams, and has two sources of authority ("Windows Server AD" and "Azure Active Directory (self-service)"), I could run the commands with no problem. However, after forcing the inicial sync, the account still has the same two sources of authority.

2. With my personal, named account, which currently is shown double on AAD(alvaro@company.com linked to Azure AD, and alvaro1234@company.onmicrosoft.com linked to on-premise AD), when I ran the command, I got the following error:

PS C:\Users\Administrator> Set-MsolUser -UserPrincipalName alvaro@company.com -ImmutableId "BuoO8NjJF0aSXA2p5e8j1A=="
Set-MsolUser : Uniqueness violation. Property: SourceAnchor.
At line:1 char:1
+ Set-MsolUser -UserPrincipalName alvaro@company.com -ImmutableId ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : OperationStopped: (:) [Set-MsolUser], MicrosoftOnlineException
+ FullyQualifiedErrorId : Microsoft.Online.Administration.Automation.UniquenessValidationException,Microsoft.Onlin
e.Administration.Automation.SetUser

I suppose this error is because, in AAD, the account alvaro@company.onmicrosoft.com is already linked to that ImmutableId. How can I handle it?