There is $table_prefix variable in wp-config.php. We
set it to '@@DB_MAIN_PREFIX@@wp_'. However, customer can change it later, and
this customer changed table prefix in his WP instance. After that customer
upgraded to next WordPress version, and upgrade failed because we re-write
configuration file on upgrade, and change prefix again, while customer tables
had different prefix.
We have to get the table prefix from wp-config.php where customer can change the table prefix and when upgrading that new version we need to upgrade with the same prefix which is in the Database, not with the Default prefix wp_table name.