On all supported storage engines, 3.4.x versions will set a bit in the data files when either an index or a collection is created with a non-simple collation. On downgrade, this bit will cause 3.2 and older versions of the server to shut down with a MustUpgrade error. After creating indexes or collections with a non-simple collation on a 3.4.x version, the following steps must be taken in order to downgrade to 3.2:
- Drop all indexes or collections with a non-simple collation.
- Downgrade to 3.2.8 or a newer 3.2 patch release.
3.2.8 contains a change which will cause the server to examine the catalog metadata if the collation feature bit is set. If no collation-related metadata associated with collections or indexes is found in the catalog, then the data files are clean for downgrade. In this case, the collation feature bit is cleared and startup will succeed.
It is not possible to downgrade to a version older than 3.2.8 after creating indexes or collections with a non-simple collation.