Thanks for the post! TestRail wouldn’t directly support exporting or importing entire projects via CSV or XML at this time and only importing test cases would be supported. You would be able to export any test runs, plans, or suites into CSV or XML, however only the test cases would be importable.
That being said, here are a couple of options you may find useful:
If you are wanting to export your projects from the same instance into a new, empty TestRail instance, you can instead backup the full database and restore it in the new instance (and perform the upgrade to TestRail 5.6). With the full migration complete, you can then delete the extra projects and deactivate any users which are not needed.
If the destination project is not empty, then you would need to recreate any data in the destination instance by using the XML or CSV export/import features or using the API to automatically create the data in the new instance. Please keep in mind that you would also need to recreate any custom fields in the destination instance to ensure the data is copied over correctly.
I hope this helps,