Index: wp-includes/class-wp-xmlrpc-server.php
===================================================================
--- wp-includes/class-wp-xmlrpc-server.php	(revision 24972)
+++ wp-includes/class-wp-xmlrpc-server.php	(working copy)
@@ -4424,19 +4424,22 @@
 		$post_author = $postdata['post_author'];
 
 		// Only set the post_author if one is set.
-		if ( isset($content_struct['wp_author_id']) && ($user->ID != $content_struct['wp_author_id']) ) {
-			switch ( $post_type ) {
-				case 'post':
-					if ( !current_user_can('edit_others_posts') )
-						return(new IXR_Error(401, __('You are not allowed to change the post author as this user.')));
-					break;
-				case 'page':
-					if ( !current_user_can('edit_others_pages') )
-						return(new IXR_Error(401, __('You are not allowed to change the page author as this user.')));
-					break;
-				default:
-					return(new IXR_Error(401, __('Invalid post type')));
-					break;
+		if ( isset($content_struct['wp_author_id']) ) {
+			// Check permissions if attempting to switch author to or from another user
+			if ( ( $user->ID != $content_struct['wp_author_id'] ) || ( $user->ID != $post_author) ) {
+				switch ( $post_type ) {
+					case 'post':
+						if ( !current_user_can('edit_others_posts') )
+							return(new IXR_Error(401, __('You are not allowed to change the post author as this user.')));
+						break;
+					case 'page':
+						if ( !current_user_can('edit_others_pages') )
+							return(new IXR_Error(401, __('You are not allowed to change the page author as this user.')));
+						break;
+					default:
+						return(new IXR_Error(401, __('Invalid post type')));
+						break;
+				}
 			}
 			$post_author = $content_struct['wp_author_id'];
 		}