def reversed_change_dict(change):
    """
    Return the dictionary of genre changes, whereby keys and values have
    swapped their roles.

    Parameters:

      change (dict): Original dictionary of genre changes, with:
        * key (string): desired genre.
        * value (list of string): None, or list of genres to be changed to
          desired genre. None counts as an empty list.

    Returns:

      dict: Reversed dictionary of genre changes, with:
        * key (string): genre to be changed.
        * value (list of string): list of desired genres.
    """
    change_rev = dict()
    for desired_genre in change:
        org_genres = change[desired_genre]
        if org_genres:
            for org_genre in org_genres:
                if org_genre not in change_rev:
                    change_rev[org_genre] = []
                change_rev[org_genre].append(desired_genre)
    return change_rev