The beauty of going to groups.io is that the list owner can invite all existing members to join the new version with one simple click by them.
If attachments don't transfer to the new archive, the old doesn't have to be deleted: simply lock it for fresh posting, and leave it sitting as a searchable archive just to infuriate the appallingly-arrogant yahell management. However, AFAIK (lazy typing) attachments do transfer.
I am lucky: a small specialist group which I own still has emails and photos going through. If/when I am forced to relocate, I will go to groups.io.