@classmethod
def transform_update_frequency(cls, frequency):
    """Get numeric update frequency (as string since that is required field format) from textual representation or
        vice versa (eg. 'Every month' = '30', '30' or 30 = 'Every month')

        Args:
            frequency (Union[str, int]): Update frequency in one format

        Returns:
            Optional[str]: Update frequency in alternative format or None if not valid
        """
    if isinstance(frequency, int):
        frequency = str(frequency)
    return cls.update_frequencies.get(frequency.lower())
