It depends on how the custom modules were developed. Most custom modules built for older Odoo versions will not work directly with Odoo 19 without testing and, in many cases, some code updates. With the Odoo 19 release, Odoo introduces changes to its framework, APIs, models, views, and security rules. If your custom module uses features that have changed, it may need to be migrated to ensure compatibility. Simple modules with minimal customization may require only minor updates, while complex modules with custom workflows, third-party integrations, or advanced business logic may need more extensive modifications. Before upgrading, it is recommended to test all custom modules in a staging environment, review dependencies, and fix any compatibility issues. This helps ensure that your Odoo 19 ERP features continue to work smoothly and allows you to take advantage of What's New in Odoo 19, including the latest Odoo 19 AI features and performance improvements, without disrupting your business operations.