Message numbers will be maintained, and attachments (if any) will be retained.
Our Yahoo messages start with number 1. There are no messages in our GIO group. The Yahoo messages will be uploaded in sequence. Hence GIO message numbers will be created to match the Yahoo message numbers.
From time to time I had pruned the Yahoo group--there are missing messages. Some were notices for meetings long past. Some were "Me too!" messages. And there were a lot of inadvertent Replies to Group that were intended to be Replies to Sender. The uploader keeps the message numbers in sync by uploading an empty message for each missing message. This should serve to assure that the new message numbers assigned by GIO match the Yahoo message numbers. It's one of the things I'm testing.