From v8.20 onwards, will expose the following extra fields in change events by default, which were previously only expanded when showExpandedEvents was set. These fields will be emitted unconditionally, regardless of the value of showExpandedEvents:

• collectionUUID for DML events (insert, update, replace, delete)
  commitTimestamp for DML events that are part of a prepared / distributed transaction
• updateDescription.disambiguatedPaths for update events

The following fields will be shown if emitting the respective events is also enabled:

• collectionUUID for DDL and control events (except invalidate, dropDatabase, endOfTransaction)
• operationDescription for DDL events (create, rename, createIndexes, startIndexBuild, abortIndexBuild, dropIndexes, modify)
• nsType for DDL create events

There will be no way to disable emitting the extra fields in change stream events in 8.2 and higher. If the fields are not needed by consumers, they can be projected out of the events using a $project stage with specific field exclusions.

During upgrade the following applies:

• shards/nodes that are running v8.2 will emit the additional fields unconditionally.
• shards/nodes still running versions older than v8.2 will only emit the additional fields if showExpandedEvents is enabled for the change stream.
• mongos will merge the streams as they are, meaning the merged stream can contain events with the additional fields (from upgraded nodes) and without the additional fields (from not-yet upgraded nodes).

  Description of Linked Ticket

  In PM-1950, we added a number of new change events, while also adding some new fields to existing events. In order to prevent disruption to users, we guarded both the new events and the new fields behind the opt-in showExpandedEvents flag. However, in the case of the new fields, this was likely an unnecessarily cautious approach; adding new fields, as opposed to modifying existing fields, does not violate the principle of the stable API. In fact, we already exposed the new wallTime field, which was originally behind the showExpandedEvents flag, in SERVER-65022.

We should reconsider whether it makes sense to move the remaining fields out from behind the expanded events flag, while continuing to keep the new events themselves behind it. The fields in question are:

• The collectionUUID field for applicable DML and DDL events.
• The disambiguatedPaths field for update events.
• The operationDescription field for pre PM-1950 events (specifically rename).