Use a copy of a real older install: for example Oqtane 6.x / 2sxc 20.x or 21.00/21.05 with existing content.

Steps:

1. Upgrade Oqtane and 2sxc using the intended order.
2. Check folder migration.
3. Load existing master sites.
4. Load existing separate-DB tenant site.
5. Add a new Content module on the separate-DB tenant.
6. Preflight | 2sxc - Content Management for DNN & Oqtane | azing.org

Expected:

• Old folders are migrated into 2sxc\Tenants\1\Sites\....
• Existing content still renders.
• Separate-DB tenant does not throw The given key '5' was not present in the dictionary.

If clean multi-DB passes but this fails, the remaining problem is likely upgrade/migration/repair, not base runtime support.