To make sure that you do not lose any of your work, all you need to be able to revert to 9.0.5 is the latest 9.0.5 code and a 9.0.5 backup-database. So first:
Then open your project in 9.9.2 (although I recommenc 9.12.4, since that is the current highest LTS), save and commit. Deploy to a test-environment and test it until you are confident that no problems arise. Then deploy to production.
Mind you: once your production-version is live, going back to 9.0.5 is almost always not possible without losing all the data-changes since going live with 9.9.2. That’s why you need to do the tests beforehand.
Note: The above screenshots are with different Mendix versions. Make sure you’re opening with Mendix version 9.9.2
Hope this helps you!
just open your mendix project in mendix studio pro 9.9.2, a popup should appear asking you to convert the project, click yes.
Then you just need to test your project to be sure everything is working as before.