How do I make a dynamically changing menu?

Create the menu with all the items that it will ever contain, then dynamically remove and insert the items as required. After loading menu, Query and maintain a copy of the menuitem(s) that will be removed.

 // Obtain and keep a copy of the convert submenuitem
  if (!WinSendMsg (pwd->hwndAB, MM_QUERYITEM,
      MPFROM2SHORT (IDM_KanCnvMnu, TRUE), MPFROMP (&pwd->miCnvMnu))) {

  // And the convert submenu text
  if (!WinSendMsg (pwd->hwndAB, MM_QUERYITEMTEXT,
    MPFROM2SHORT (IDM_KanCnvMnu, sizeof (pwd->szCnvMnuTxt)),
                  MPFROMP (&pwd->szCnvMnuTxt)) ){

When menu is to updated, insert/remove as necessary

    // This is layout, is the submenu already in place?
    if (!pwd->fCnvMenu) {

      // The submenu is not installed, so insert the submenu
      WinSendMsg (pwd->hwndAction, MM_INSERTITEM,
                  MPFROMP (&pwd->miCnvMnu), MPFROMP (pwd->szCnvMnuTxt));

      // And remove the convert menuitem
      WinSendMsg (pwd->hwndAB, MM_REMOVEITEM,
                  MPFROM2SHORT (IDM_KanCnv, TRUE), 0L);

      // Set the submenu flag
      pwd->fCnvMenu = TRUE;
    }

Credit: Matthew S. Osborn