def compare_hash_tables(cls):
    """ Return a list of nc files that have changed or are new """
    old_hashes = cls.get_old_hashes()
    changed = []
    if old_hashes:
        if len(cls.new_hashes) == 0:
            cls.load_hash_table()
        for key in cls.new_hashes:
            if key not in old_hashes.keys() or key in old_hashes.keys(
                ) and old_hashes[key] != cls.new_hashes[key]:
                changed.append(key)
    return changed
