Set-SqlServiceAccount.ps1

#HT http://blogs.technet.com/b/heyscriptingguy/archive/2010/11/03/use-powershell-to-change-sql-server-s-service-accounts.aspx
[System.Reflection.Assembly]::LoadWithPartialName('Microsoft.SqlServer.SqlWmiManagement')| Out-Null
function Set-SqlServiceAccount{
    param([string[]] $Instance
        ,[System.Management.Automation.PSCredential]$ServiceAccount
        ,[ValidateSet('SqlServer','SqlAgent')] $service = 'SqlServer'
    )
    
    #Get Service Account name from credential object
    $account =$ServiceAccount.GetNetworkCredential().Domain +'\'+ $ServiceAccount.GetNetworkCredential().UserName
    
    #Loop through and change instances
    foreach($i in $Instance){
        #Parse host and instance names
        $HostName = ($i.Split('\'))[0]
        $InstanceName = ($i.Split('\'))[1]

        #Get service account names, set service account for change
        $sqlsvc = if($InstanceName){"MSSQL`$$InstanceName"}else{'MSSQLSERVER'}
        $agtsvc = if($InstanceName){"SQLAGENT`$$InstanceName"}else{'SQLSERVERAGENT'}

        $ServiceName = switch($service){
            'SqlServer'{$sqlsvc}
            'SqlAgent'{$agtsvc}
        }

        #Use wmi to change account
        $smowmi = New-Object Microsoft.SqlServer.Management.Smo.Wmi.ManagedComputer $HostName
        $wmisvc = $smowmi.Services | Where-Object {$_.Name -eq $ServiceName}
        $wmisvc.SetServiceAccount($account,$ServiceAccount.GetNetworkCredential().Password)
    
        #If Agent service isn't running (happens if you reset SQL Service), start it
        $wmiagt = $smowmi.Services | Where-Object {$_.Name -eq $agtsvc}
        if($wmiagt.ServiceSatus -ne 'Running'){$wmiagt.Start()}
    }
}


<#
$cred = Get-Credential 'Enter Service Account'
Set-SqlServiceAccount -Instance PICARD -ServiceAccount $cred -Service SqlServer
#>