The answer is pretty much given already. I will summarize it with some extra details:
- When you are ready, stop your production application to prevent it from creating new data
- manually trigger a backup action, this creates a snapshot of the current database + files
- download the full backup which includes both files and database
- inform mendix support to down scale your app/database
- after down scaling has been completed, upload your full backups archive
- restore the uploaded archive to your environment
- start environment and confirm your data is still there
With this approach, your SAML configuration will be kept. Because the related data is stored in database and files which will be restored.
The above instructions should be given by Mendix support already.
Personally I would advice you to practice the procedures first with non-production to get familiar with the steps.