Unless things have changed in recent years, if you unpublish a process, it will not cancel the active instances; however, User Tasks for that process will become inaccessible to standard users.

Usually, the best way to retire a process is to revoke submitter access and change the URL so no new instances are created, then wait until all active instances are complete before unpublishing.

Using current form link for new/modified form - Laserfiche Answers