(based on a true story)
Step 0. Find an existing low-volume announcements-only mailing list, e.g. that of a certain scientific software.
Step 1. Migrate the users to a new mailing list meant for both announcements and general discussions.
Step 2. Make the new mailing list unmoderated and set the Reply-To address to be the list submission address.
Step 3. Post the announcement about the change on the list itself.
Step 4. Realise that many users have completely forgotten that they were on the former list (because it was a really low-volume one).
Step 5. Watch as angry unsubscribe messages (mostly from people who have already forgotten about the software) start hitting the list and result in a quickly growing avalanche of angry unsubscribe requests.
Step 6. Realise that step 2 was a really really really dumb one.
Step 7. Realise that step 1 was probably dumb too.
Step 8. Unsubscribe everyone from the new mailing list.
Step 9. Spend some time cleaning up your inbox.
Optional (recommended): Let an experienced list administrator show you how to do it properly with two separate lists. It’s not like your server can’t handle two instead of one, is it?
Never let scientists do the work of the system administrators!