+-----------------------------------------------------------------+
|                                                                 |
|                           Decoded Text                          |
|                                                                 |
|                                                                 |
|  +--------------------+    downgrade    +--------------------+  |
|  | Internally encoded | --------------> | Internally encoded |  |
|  | as UTF-8           |                 | as iso-8859-1      |  |
|  | (is_utf8 = 1)      | <-------------- | (is_utf8 = 0)      |  |
|  +--------------------+     upgrade     +--------------------+  |
|                                                                 |
+-----------------------------------------------------------------+
                             |       ^    
                             |       |
                      encode |       | decode
                             |       |
                             v       |
+-----------------------------------------------------------------+
|                                                                 |
|                             Bytes or                            |
|                           Encoded Text                          |
|                                                                 |
|                                                                 |
|  +--------------------+    downgrade    +--------------------+  |
|  | Internally encoded | --------------> | Internally encoded |  |
|  | as UTF-8           |                 | as iso-8859-1      |  |
|  | (is_utf8 = 1)      | <-------------- | (is_utf8 = 0)      |  |
|  +--------------------+     upgrade     +--------------------+  |
|                                                                 |
+-----------------------------------------------------------------+