PostInstallScript.cls

global class PostInstallScript implements InstallHandler {
    global void onInstall (InstallContext context)
    {
        // Only initialize package on a fresh install!
        if (context.previousVersion == null)
        {
            InitializePackage();
        }
        // Migrate data, but only for versions older than 1.2
        else if (context.previousVersion().CompareTo(new Version (1,2)) < 0)
        {
            UpgradeForFeature12();
        }
    }
}