Function Fix-O365UPNMisMatch
{
    [CmdletBinding()]
    Param
    (
        [Parameter(Mandatory=$true)]
        $UserAzureUPN
        ,
        [Parameter(Mandatory=$true)]
        $UserNewAzureUPN
        ,
        [Parameter(Mandatory=$true)]
        $UserTempAzureUPN
 
    )
 Connect-MsolService
    $user = Get-MsolUser -UserPrincipalName $UserAzureUPN -EA STOP
 
    try
    {
        Set-MsolUserPrincipalName -ObjectId $user.ObjectId -NewUserPrincipalName $UserTempAzureUPN -EA STOP
 
        try
        {
            Set-MsolUserPrincipalName -ObjectId $user.ObjectId -NewUserPrincipalName $UserNewAzureUPN -EA STOP
        }
        catch
        {
            "Could not set $($user.UserPrincipalName) to new UPN: $UserNewAzureUPN EXCEPTION: $($_.Exception.Message)"
        }
    }
    catch
    {
        Write-Host "Could not set $($user.UserPrincipalName) to temporary UPN: $UserTempAzureUPN EXCEPTION: $($_.Exception.Message)"
    }
}

Fix-O365UPNMisMatch -UserAzureUPN "oldupn@mycompany.com" -UserTempAzureUPN "oldupntemp@mytennant.onmicrosoft.com" -UserNewAzureUPN newupn@mycompany.co.uk"