Hi,
The Feature Compatibility Version (FCV) is set as the last step in a major version upgrade as it enables features that can persist data incompatible with earlier versions of MongoDB. Incompatible data types will definitely complicate the downgrade process as you will need to modify or remove data that is unsupported by older versions of MongoDB.
The Release Notes for every major MongoDB server release include a detailed section on Compatibility Changes and the downgrade instructions also mention incompatible features. For example: Remove FCV 5.0 Persisted Features.
If you need time to validate a newly upgraded deployment you should wait before enabling the new FCV version, per the note in the docs:
It is recommended that after upgrading, you allow your deployment to run without enabling these features for a burn-in period to ensure the likelihood of downgrade is minimal. When you are confident that the likelihood of downgrade is minimal, enable these features.
Regards,
Stennie