In your existing app: 

  1. In your current app, Create a new backup.
    1. Go to Settings > Backup & Restore.
    2. Tap Create new backup. A new backup is created and displayed under Restore from backups.
    3. Tap on the new backup created (under Restore from Backups)

  2. Share the backup. It displays Share options. Tap Share via AirDrop.

  3. Tap on the Combined App icon

  4. The backup is now imported to the new app and a message is displayed about where to find the backup in the new app under settings Backup & Restore.

  5. In the new (combined) app, go to Menu > Settings > Backup & Restore > Your Backups and you will find your backup listed here.

  6. Tap on the backup that is to be restored. Tap on Replace to replace the existing default vocabulary with the backup from your previous app.

  7. Your new app now has your existing (old) app's vocabulary and is ready to use.  

Note: If you face any issues while migrating through the above method, you can also migrate it through Files via AirDrop. Refer to the link below. 

Related articles: 
How to migrate your backup to the new combined app through Files (via AirDrop) - for iOS 11-14 only.