Login domain\user’ has granted one or more permissions. Revoke the permission before dropping the login (Microsoft SQL Server, Error: 15173)

$svrs = ## list of servers Get-Content from text fiel etc

foreach ($svr in $svrs) {
    $server = New-Object Microsoft.SQLServer.Management.Smo.Server $svrs
    foreach ($endpoint in $server.Endpoints['Mirroring']) {
        if ($endpoint.Owner = 'Domain\User') {
            $endpoint.Owner = 'Domain\NEWUser'
            $endpoint.Alter()
        }        
    }
}