How to change the existing Window Membership Provider to Claims Based Authentication in SharePoint 2010?

$WebAppName = "http://yourWebAppUrl"
$wa = get-SPWebApplication $WebAppName
$wa.UseClaimsAuthentication = $true
$wa.Update()


$account = "yourDomain\yourUser"
$account = (New-SPClaimsPrincipal -identity $account -identitytype 1).ToEncodedString()
$wa = get-SPWebApplication $WebAppName
$zp = $wa.ZonePolicies("Default")
$p = $zp.Add($account,"PSPolicy")
$fc=$wa.PolicyRoles.GetSpecialRole("FullControl")
$p.PolicyRoleBindings.Add($fc)
$wa.Update()


$wa.MigrateUsers($true)


$wa.ProvisionGlobally()

Source : http://technet.microsoft.com/en-us/library/gg251985.aspx

Extend a Web application that uses Windows-claims authentication (SharePoint Server 2010)