It is strongly recommended to take an export of your application before beginning the migration process.
This will enable you to easily revert back to the original application if necessary, before making any changes to your application and its metadata.
Once you have migrated to the Universal Theme, you can not easily switch it back to the original legacy theme.
Additionally, if you re-import the original application, under a different Application ID, you can refer to this application to review how the application looked previously, as opposed to how it looks using the Universal Theme.
If you haven't done so already, you should:
Use the Application Utilities > Upgrade Application to update certain built-in widgets in your legacy applications to the latest provided by APEX 5, e.g. FlashChart to AnyChart.
This will ensure that those widgets take full advantage of the responsiveness of Universal Theme.