def replace_tags(string, from_tag='i', to_tag='italic'):
    """
    Replace tags such as <i> to <italic>
    <sup> and <sub> are allowed and do not need to be replaced
    This does not validate markup
    """
    string = string.replace('<' + from_tag + '>', '<' + to_tag + '>')
    string = string.replace('</' + from_tag + '>', '</' + to_tag + '>')
    return string