When upgrading an old application to the latest version of 4D, it is recommended to upgrade in-between versions as much as possible. For example, if an application developed with v13 needs to be upgraded to v19, ideally, the upgrade process should go v13 —> v14 —> v15 —> … —> v19. This way, any issues that arise can be more easily managed. For example, it reduces the amount of deprecated items to update at each step, so issues like obsolete commands can be taken care of gradually. Skipping some versions is still possible, but it is not recommended to jump straight from v13 to v19, for example.