If you want to change the default behavior of a module classes and be sure that these changes won't be overwritten during the module upgrade process, you should **copy the original classes** that you are going to customize to the `prestashop-root/modules/retailcrm_custom/classes` directory.
From here you can modify the methods of the classes for your own purposes, and they will not be affected during the module upgrade process.
Keep in mind that:
* If the logic and classes of the module have changed a lot after an upgrade, your customized logic may cause the module to malfunction. **You should always check for changes after an upgrade and update your customized classes if needed.**
* This feature does not allow to customize the base class (file `retailcrm.php`). For this you can use the standard Prestashop override feature.
