Personally, I set up an automation thru Airtable and Zapier to do this. It’s a pretty major workaround, but it essentially recreates the task and assigns it to the new owner, then deletes the original task. Airtable holds a backup so nothing is lost. If this gets reply gets 50 upvotes, I will write out a tutorial with screenshots and publish it on Medium or something.
Unfortunately my workaround will become much more complicated for most users once Hubspot forces us to use private apps instead of account API keys 