Sets the UIVersion to v3 or v4 for all sites in a given database

SetSPUIVersion.ps1

function Set-SPUIVersion(
    [string]$dbName = $(Read-Host -prompt "Content Database Name"),
    [int32]$uiVersion = $(Read-Host -prompt "UI Version"))
    {
    $db = Get-SPContentDatabase $dbName
 
    foreach ($s in $db.Sites) {
        foreach ($w in $s.AllWebs) {
            $w.UIversion = $uiVersion;
            switch ($uiVersion) {
                3 { $w.UIVersionConfigurationEnabled = $true; }
                4 { $w.UIVersionConfigurationEnabled = $false; }
            }
            $w.Update();
        }
    }
}