A channel_id_changed event is sent when a user initiates sharing a private channel externally with Slack Connect. When a private channel is shared, the existing private channel's unique channel ID changes to a new identifier permanently. This event is only sent for private channels your app has access to.
When receiving this event, you should update any records you have stored about the original private channel ID (old_channel_id) with this new_channel_id. From now on, all messages and other happenstance around the channel will be associated with the new ID. If you attempt to post messages, perform lookups, or other operations with the channel you will need the channel's new ID.
While there are other reasons a channel ID may change, this event currently applies to only the circumstances with Slack Connect described above.