Remove post_tag from default post type, add custom taxonomy

to completely remove post tags: https://stackoverflow.com/a/8363082

if you want to remove post tags from posts, but keep on other custom post types:

add_action( 'init', 'my_register_post_tags' );

function my_register_post_tags() {
    register_taxonomy( 'post_tag', array( 'my_post_type_here' ) );
}

to add your own custom taxonomy to posts, use register_taxonomy, setting the $object_type parameter to post.