Feature13Installer.cls

public class Feature13Installer implements InstallHandler {

    global void onInstall (InstallContext context)
    {
        if (context.previousVersion() == null)
        {
            InitializePackage();
        }
        else if (context.previousVersion().CompareTo(new Version (1,3)) < 0)
        {
            UpgradeForFeature13();
        }
    }
}