updating credentials using powershell

I am in a position where service credentials change quite a lot, I would normally use central admin to change the credentials but quite often the application pool that runs that is using the same credentials which I want to change.

I've tried writing the following powershell script but I always get FALSE as the result of SetPassword. Any ideas what i am doing wrong?

$login and $password are parameters which are passed in to the script.

$managedAccount = Get-SPManagedAccount
$secureString = convertto-securestring $password -asplaintext -force
##Set-SPManagedAccount -Identity $managedaccount -ConfirmPassword $securestring -NewPassword $secureString

$Apps = Get-SPWebApplication
foreach ($App in $Apps)
{
    $AppPool = $App.ApplicationPool
    #$AppPool
    #Write-Host "$($AppPool.Username) $($login) $($AppPool.Username -eq $login)"
    if ($App.ApplicationPool.Username -eq $login)
    {
        write-host "setting credentials"
        $App.ApplicationPool.CurrentIdentityType= [Microsoft.SharePoint.Administration.IdentityType]::SpecificUser
        $App.ApplicationPool.Username = $login
        $result = $App.ApplicationPool.SetPassword($securestring) 
        $result

        # Save the settings
        $App.ApplicationPool.UpdateCredentials($login)

        # Roll the settings out via a Admin Job
        $App.ApplicationPool.Provision()
    }
}