This talk is inspired from a real-life project, and it is a primer for anyone interested in migrating legacy APEX application developed in APEX 3.x or APEX 4.x to APEX 19.2. We will go through every step we took to successfully migrate the whole portfolio of about 200 APEX applications, from 4.x to 19.2. APEX 5.0, released in 2015, introduced the Universal Theme (UT), defined as "a responsive, versatile, and customizable user interface for your Application Express apps". All new APEX applications, developed with at least version 5.0, benefit from using Universal Theme. But what about the legacy applications, developed in APEX 3.x or 4.x? If the applications work perfectly, should I migrate them to APEX 19.2 and their templates to the new UT ? What are the risks? Is the migration process flawless? What can go wrong? Can I rollback if I don't like the result? Do I need any additional work when migrating the application to APEX 19.2 ? This presentation aims to give answers to these questions, answers that we found in the most practical way: during our migration projects.