Failing to upgrade from 3.1.1-4 to any version above due to null values

Hi @jascanio
I am trying to upgrade from

php tools/upgrade.php check
Code version:      2.4.8.3
Database version:  2.4.8.3
Latest version:    3.3.0.8

to 3.0.2 and I have similar ERROR

php tools/upgrade.php check
Code version:      3.0.2.0
Database version:  2.4.8.3
Latest version:    3.3.0.8
Database version is older than code version
Run "tools/upgrade.php upgrade" to update

ERROR: Upgrade failed: DB: Incorrect string value: '\xD9\x86\xD8\xB4\xD8\xA7...' for column 'subject' at row 1

In this post is a posible solution but I tried and didn’t work. I double check config.inc.php, I add and remove options, I tried other versions like 3.2.1-3 and 3.3.0-8, but nothing works. There has to be something I am misunderstanding.

mysql> SHOW VARIABLES LIKE 'char%';
+--------------------------+----------------------------+
| Variable_name            | Value                      |
+--------------------------+----------------------------+
| character_set_client     | utf8mb4                    |
| character_set_connection | utf8mb4                    |
| character_set_database   | utf8mb4                    |
| character_set_filesystem | binary                     |
| character_set_results    | utf8mb4                    |
| character_set_server     | utf8mb4                    |
| character_set_system     | utf8mb3                    |
| character_sets_dir       | /usr/share/mysql/charsets/ |
+--------------------------+----------------------------+
8 rows in set (0.04 sec)

mysql> SHOW VARIABLES LIKE 'collat%';
+----------------------+--------------------+
| Variable_name        | Value              |
+----------------------+--------------------+
| collation_connection | utf8mb4_0900_ai_ci |
| collation_database   | utf8mb4_0900_ai_ci |
| collation_server     | utf8mb4_0900_ai_ci |
+----------------------+--------------------+
3 rows in set (0.00 sec)

mysql> 

I tried with this too, but I get the same error:

mysql> CREATE DATABASE ojs CHARACTER SET utf8 COLLATE utf8_general_ci;

It is necessary to “convert” DB content to UTF8 or something like that? How can I do that?

Thanks