PostInstallScript.cls

global class PostInstallScript implements InstallHandler {

    global void onInstall (InstallContext context)
    {
        // Initialize package (and new feature) on fresh installs
        if (context.previousVersion() == null)
        {
            InitializePackage();
            InitializeFeature13();
        }
        // Migrate data, but only for versions older than 1.2
        else if (context.previousVersion().CompareTo(new Version (1,2)) < 0)
        {
            UpgradeForFeature12();
            UpgradeForFeature13();
        }
        // Migrate existing data, for versions older than 1.3
        else if (context.previousVersion().CompareTo(new Version (1,3)) < 0)
        {
            UpgradeForFeature13();
        }
    }
}