解決方案：在AppDelegate.h中的didFinishLaunchingWithOptions裡面添加：

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    // Override point for customization after application launch.
    NSLog(@did Finish Launching With Options);
    
    if ([[NSUserDefaults standardUserDefaults] boolForKey:@isFirstLaunched]){
        NSLog(@AppDelegate - didFinishLaunchingWithOptions: App was already launched.);
    }
    else{
        NSLog(@AppDelegate - didFinishLaunchingWithOptions: App's first launch.);
        
        [[NSUserDefaults standardUserDefaults] setBool:YES forKey:@isFirstLaunched];
        [[NSUserDefaults standardUserDefaults] synchronize];
    }
    
    if ([[[NSUserDefaults standardUserDefaults] stringForKey:@clientVersion] isEqualToString:CLIENT_VERSION]) {
        NSLog(@AppDelegate - didFinishLaunchingWithOptions: App version remains the same.);
    }
    else{
        NSLog(@AppDelegate - didFinishLaunchingWithOptions: App version did update.);
        [[NSUserDefaults standardUserDefaults] setObject:CLIENT_VERSION forKey:@clientVersion];
        [[NSUserDefaults standardUserDefaults] synchronize];
    }
    
    return YES;
}

在App-Prefix.pch中定義CLIENT_VERSION：

    #define CLIENT_VERSION [[[NSBundle mainBundle] infoDictionary] objectForKey:@CFBundleShortVersionString]