Index: wp-admin/includes/upgrade.php
===================================================================
--- wp-admin/includes/upgrade.php	(revisión: 32424)
+++ wp-admin/includes/upgrade.php	(copia de trabajo)
@@ -1759,10 +1759,31 @@
 		}
 	}
 
-	return $wpdb->query( "ALTER TABLE $table CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci" );
+	if (_get_table_collation($table) != 'utf8mb4_unicode_ci')
+		return $wpdb->query( "ALTER TABLE $table CONVERT TO CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci" );
+
+	return true;
 }
 
 /**
+ * Get the collation of a table utf8mb4.
+ *
+ * @since 4.2.0
+ *
+ * @param string $table The table to check.
+ * @return string like utf8mb4_unicode_ci.
+ */
+function _get_table_collation($table) {
+    global $wpdb;
+
+    $table_detail = $wpdb->get_results("show table status like '".$table."'");
+    if (empty($table_detail)) return false;
+    $table_detail = array_shift($table_detail);
+
+    return $table_detail->Collation;
+}
+
+/**
  * Retrieve all options as it was for 1.2.
  *
  * @since 1.2.0