Upgrading replica set from 3.4 to 3.6 fails when a user-defined database name conflicts with the "config" database that needs to be created during upgrade. (See SERVER-41350).
This is definitely the case where there is a case mismatch (e.g., pre-existing database is named "Config" or "CONFIG") – when the pre-existing database is named "config" this is still incorrect, but the upgrade will probably succeed.
Customers should be advised to check for and resolve naming conflicts with config database or config.sessions and config.transactions collections before attempting to upgrade.
- Update release notes upgrade guides to call this out
- Backport work for DOCS-12757 to 3.4