Answer

Versions 18.3 and later

When comment flow is enabled on either a new or a running integration, Hub will now flow existing comments on relevant artifacts and any new comments that are created. Previously, if you enabled comment flow on an already-running integration, only new comments would flow and existing comments would be ignored.

The same workflow is true for Attachment flow.

Versions 17.1 - 18.2:

If you enable comment flow before you turn on your integration for the first time, all existing comments will sync over as part of your integration, as well as any new comments made after the integration begins running.

If you enable comment flow after your integration has already been running for a while, only comments made after comment flow is enabled will sync over.

The same is true for Attachment flow.

Clicking Process All Artifacts will not flow over old comments or attachments.