View_Ciao *viewCiao;
-(void) viewDidLoad {
  viewCiao = [[View_Ciao alloc] initWithNibName:@"View_Ciao" bundle:nil];
  [viewCiao.view setNeedsDisplay];
  [super viewDidLoad];
}

- (void)applicationDidFinishLaunching:(UIApplication *)application {

    [window addSubview:viewController.view];
    [window makeKeyAndVisible];

    // create the second view, remove the first, and then display the second

    viewCiao = [[ViewCiao alloc] init];
    [viewController.view removeFromSuperview];
    [window addSubview:viewCiao.view];
}

@interface AppDelegate : ...
{
    View_Hello *hello;
    View_Goodbye *goodbye;
    View_Ciao *ciao;
    UIViewController *currentView;
    UIWindow *window;
}
... IBOutlet properties for the 3 controllers.
@end

@implementation AppDelegate
- (void)switchToGoodbye
{
    [currentView.view removeFromSuperview];
    [window addSubview:goodbye.view];
    currentView = goodbye;
}
... etc.
@end