Odoo is designed to be backward compatible, so you should be able to use your existing custom modules in Odoo 16. However, there may be some changes or updates to the Odoo platform in version 16 that could affect the way your modules work.

It's always a good idea to test your existing custom modules in a development environment before upgrading to a new version of Odoo. This will give you a chance to identify and address any compatibility issues before you upgrade your production system.
If you encounter any compatibility issues with your existing custom modules in Odoo 16, you may need to update your modules to work with the new version. Odoo provides extensive documentation and resources for developers, so you can refer to the official documentation to learn more about the changes in Odoo 16 and how to update your modules accordingly.