Before you start an upgrade to the latest 12c release, it is recommended that you create copies of existing Forms template files. This will help you to restore user-specific customization in the new forms template files.
This includes all files in the following directory:
<ORACLE_HOME>\user_projects\domains\base_domain\config\fmwconfig\components\FORMS\instances\<INSTANCE NAME>\server
If you are upgrading from a previous 12c release to the latest release, the files are in the same directory as mentioned above. If you are upgrading Oracle Forms from a release earlier than 12c (for example, 11g) to the latest 12c release, the files may be in a different directory.
After completing the upgrade process, you should manually copy or merge user-specific customizations created in the earlier version to the new Forms template files.