Cron job to change CPT

Try something like this:

if (!wp_next_scheduled('update_members_types')) {
    wp_schedule_event( time(), 'daily', 'update_members_types' );
}
add_action ( 'update_members_types', 'update_member_post_type' );

function update_member_post_type() {

    $args = array(
        'post_type' => 'member',
        'posts_per_page' => '1',
        'date_query' => array(
            array(
                'after' => strtotime( '-24 hours' )
            )
        )
    );
    $members = get_posts( $args );

    if ( $members ) {
        foreach ( $member as $members ) {
            set_post_type( $member->ID, 'post' );
        }
    }

} 

WordPress has a function which allows you to update the post type for the post ID.

https://developer.wordpress.org/reference/functions/set_post_type/

Note that the cron task is not scheduled to run per say at midnight each day but every 24 hours from the time of initialisation.