Index: src/wp-admin/post-new.php
===================================================================
--- src/wp-admin/post-new.php	(revision 48947)
+++ src/wp-admin/post-new.php	(working copy)
@@ -78,6 +78,9 @@
 	// Flag that we're not loading the block editor.
 	$current_screen = get_current_screen();
 	$current_screen->is_block_editor( false );
+
+	/** This filter is documented in wp-admin/post-new.php */
+	do_action( 'replaced_editor', $post );
 }
 
 require_once ABSPATH . 'wp-admin/admin-footer.php';
Index: src/wp-admin/post.php
===================================================================
--- src/wp-admin/post.php	(revision 48947)
+++ src/wp-admin/post.php	(working copy)
@@ -180,6 +180,15 @@
 		 * @param WP_Post $post    Post object.
 		 */
 		if ( true === apply_filters( 'replace_editor', false, $post ) ) {
+			/**
+			 * Fires when a replacement editor is expected to load.
+			 *
+			 * @since 5.6.0
+			 *
+			 * @param WP_Post $post The post that is being edited.
+			 */
+			do_action( 'replaced_editor', $post );
+
 			break;
 		}