Feature12Installer.cls

public class Feature12Installer implements InstallHandler {

    global void onInstall (InstallContext context)
    {
        // Migrate data, but only for versions older than 1.2
        if (context.previousVersion != null && context.previousVersion().CompareTo(new Version (1,2)) < 0)
        {
            UpgradeForFeature12();
        }
    }
}