Filter Author Bio

You can use the filters the_author_$meta and get_the_author_$meta (where $meta is in this case ‘description’):

add_filter('the_author_decription', 'custom_about_member', 10, 2);
add_filter('get_the_author_decription', 'custom_about_member', 10, 2);
function custom_about_member($description, $userid) {
    $about = get_user_meta($userid, 'user_about', true);
    if ($about) {return $about;}
    return $description;
}

This of course relies on the place where the author bio is being displayed using the related function to get the description. eg. get_the_author_meta('description') or get_the_author_description()