Hi,

Odoo regularly deprecates code in new versions, which means certain functions or modules might no longer exist or have changed significantly. The underlying architecture of Odoo might have changed, making the older code incompatible with the newer version. Newer versions of Odoo may introduce changes to the database schema, which can affect the compatibility of older code. Update the legacy code to the new version of Odoo by making necessary changes, removing deprecated functions, and updating module structures. If there are substantial changes between versions, you may use migration tools or scripts to help automate some of the code adaptation. Using legacy code might not be the most optimal approach in the long run.

The extend method is used to create a new class that inherits from an existing class and adds or overrides its methods. It allows you to extend the functionality of an existing class without modifying the original class directly. Patching is a way to modify the behavior of an existing method without overriding it completely. In OWL, patching can be achieved using the "patch" method provided by the framework.
That means patching is not used as the counterpart to extend; both are used in different scenarios. You can refer to the blogs

https://www.cybrosys.com/blog/super-override-existing-functions-using-owl
https://www.cybrosys.com/blog/how-to-patch-existing-owl-component-in-odoo-16

Hope it helps