To view status of current user interface.
$sc = Get-SPSite http://machinename/sites/collectionname; $sc.GetVisualReport() | Format-Table

To revert sites to the previous user interface.
Get-SPSite http://machinename/sites/V3UI | Get-SPWeb "webname" | Foreach{$_.UIVersionConfigurationEnabled=1;$_.UIVersion=3;$_.Update();}

To reverts all sites in a site collection to the previous user interface.
Get-SPSite http://machinename/sites/V3UI | Foreach{$_.UIVersionConfigurationEnabled=1;$_.UIVersion=3;$_.Update();}

To force through an upgrade to the new user interface.
$webapp = Get-SPWebApplication http://sitename
foreach ($s in $webapp.sites)
{$s.VisualUpgradeWebs() }

To upgrade a single site collection to the new user interface.
$site = Get-SPSite http://server
$site.VisualUpgradeWebs()

To upgrade a single site to the new user interface.
$web = Get-SPWeb http://server/site
$web.UIVersion = 4
$web.UIVersionConfigurationEnabled = 0
$web.Update()