今のところ、目的の結果を得るには、プログラムを実行する前に表示モードをフルスクリーンのボーダレス ウィンドウに変更する必要があります。プログラムの実行中に Alt + Enter を押すだけで、表示モードをウィンドウ表示からフルスクリーンのボーダレスウィンドウに変更したいと考えています。実行時に表示モードを変更できるようにコードを変更するにはどうすればよいですか?
SystemClass.cpp
void SystemClass::InitializeWindows()
{
    WNDCLASSEX wc;
    DEVMODE dmScreenSettings;
    int screenWidth, screenHeight;
    int posX, posY;
    // Get an external pointer to this object
    ApplicationHandle = this;
    // Get the instance of this application
    m_hinstance = GetModuleHandle( NULL );
    // Give the application a name
    m_applicationName = "Zero DirectX Framework";
    // Setup the windows class with default settings
    wc.style            = CS_HREDRAW | CS_VREDRAW | CS_OWNDC;
    wc.lpfnWndProc      = WndProc;
    wc.cbClsExtra       = 0;
    wc.cbWndExtra       = 0;
    wc.hInstance        = m_hinstance;
    wc.hIcon            = LoadIcon( NULL, IDI_WINLOGO );
    wc.hIconSm          = wc.hIcon;
    wc.hCursor          = LoadCursor( NULL, IDC_ARROW );
    wc.hbrBackground    = (HBRUSH)GetStockObject( BLACK_BRUSH );
    wc.lpszMenuName     = NULL;
    wc.lpszClassName    = m_applicationName;
    wc.cbSize           = sizeof( WNDCLASSEX );
    // Register the window class
    RegisterClassEx( &wc );
    // Determine the resolution of the clients desktop screen
    screenWidth  = GetSystemMetrics( SM_CXSCREEN );
    screenHeight = GetSystemMetrics( SM_CYSCREEN );
    // Setup the screen settings depending on whether it is running in full screen or in windowed mode
    if ( FULL_SCREEN )
    {
        // If full screen set the screen to maximum size of the users desktop and 32bit
        memset( &dmScreenSettings, 0, sizeof(dmScreenSettings) );
        dmScreenSettings.dmSize         = sizeof(dmScreenSettings);
        dmScreenSettings.dmPelsWidth    = (unsigned long)screenWidth;
        dmScreenSettings.dmPelsHeight   = (unsigned long)screenHeight;
        dmScreenSettings.dmBitsPerPel   = 32;
        dmScreenSettings.dmFields       = DM_BITSPERPEL | DM_PELSWIDTH | DM_PELSHEIGHT;
        // Change the display settings to full screen
        ChangeDisplaySettings( &dmScreenSettings, CDS_FULLSCREEN );
        // Set the position of the window to the top left corner
        posX = posY = 0;
        // Create the window with the screen settings and get the handle to it
        m_hwnd = CreateWindowEx( WS_EX_APPWINDOW, m_applicationName, m_applicationName, 
        WS_CLIPSIBLINGS | WS_CLIPCHILDREN | WS_POPUP,
        posX, posY, screenWidth, screenHeight, NULL, NULL, m_hinstance, NULL);
        SetMenu( m_hwnd, NULL );
    }
    else
    {
        // If windowed then set it to 800x600 resolution
        screenWidth  = 1280;
        screenHeight = 768;
        // Place the window in the middle of the screen
        posX = ( GetSystemMetrics( SM_CXSCREEN ) - screenWidth ) / 2;
        posY = ( GetSystemMetrics( SM_CYSCREEN ) - screenHeight) / 2;
        m_hwnd = CreateWindowEx( 0, m_applicationName, m_applicationName, WS_CAPTION | WS_MINIMIZEBOX | WS_SYSMENU, 
        posX, posY, screenWidth, screenHeight, 
        NULL, NULL, m_hinstance, NULL );
    }
    // Bring the window up on the screen and set it as main focus
    ShowWindow( m_hwnd, SW_SHOW );
    SetForegroundWindow( m_hwnd );
    SetFocus( m_hwnd );
    // Hide the mouse cursor
    ShowCursor(true);
}