Update status, meta while inside a post using AJAX button

First things first

In a wp_ajax_{action} or wp_ajax_nopriv_{action} callback, the global $post variable is by default a null because WordPress has not yet set it up. You can confirm it using a simple callback like so:

add_action( 'wp_ajax_test', function(){
    global $post;
    var_dump( $post );
    wp_die();
} );

Visit /wp-admin/admin-ajax.php?action=test and you should see NULL displayed on the page. If you don’t see a NULL, then it’s possible that a plugin or custom code has made a query which changed the $post variable.

Now a solution

Since you’re making the AJAX request via a button click and the button is inside a meta box which I suppose is on the post edit screen (wp-admin/edit.php), then an easy way to fix the issue of the $post being still a NULL in your wp_ajax_change_status callback, is to send the post ID from your AJAX script (JavaScript).

On the post edit screen, the post ID is stored in a hidden input field named post_ID which looks like this and stores the ID of the post that’s currently being edited:

<input type="hidden" id='post_ID' name="post_ID" value="123" />

So you can use the value and send it to your AJAX callback. E.g.:

jQuery.ajax({
    url: '/wp-admin/admin-ajax.php',
    data: {
        action:  'change_status',
        post_id: jQuery( '#post_ID' ).val() // send the post ID
    },
    type: 'post'
});

Then in your AJAX callback (i.e. the ew_change_status() function), you can retrieve the submitted post ID like so:

$post_id = filter_input( INPUT_POST, 'post_id' );

Additional Notes

1. In your AJAX callback, make sure to check whether the current user can edit the post. E.g.:

   if ( ! current_user_can( 'edit_post', $post_id ) ) {
       wp_die();
   }
   

2. Use a nonce (because you are editing a post). For example,

   a) In your meta box, create a hidden input for storing the nonce:

   // Creates an <input> with the `id` and `name` "my_security".
   <?php wp_nonce_field( 'change-post-status', 'my_security' ); ?>
   

   b) Include the nonce in the AJAX request:

   jQuery.ajax({
       url: '/wp-admin/admin-ajax.php',
       data: {
           action:  'change_status',
           post_id:  jQuery( '#post_ID' ).val(),
           security: jQuery( '#my_security' ).val()
       },
       type: 'post'
   });
   

   c) In your AJAX callback, verify the nonce:

   check_ajax_referer( 'change-post-status', 'security' );
   

So your ew_change_status() may look like:

function ew_change_status() {
    check_ajax_referer( 'change-post-status', 'security' );

    $post_id = filter_input( INPUT_POST, 'post_id' );
    if ( ! $post_id || ! ( $post = get_post( $post_id ) ) ) {
        wp_die( 'No such post, or invalid ID.' );
    }

    if ( ! current_user_can( 'edit_post', $post_id ) ) {
        wp_die( 'You can not edit this post.' );
    }

    ... your code here ...

    wp_die();
}