I would issue lots of announcements to the members before the move explaining everything, but I would set the yahoo group to announcements-only *immediately* after the transfer is complete. You domt want half the conversations in the old group and half in the new group. Any posts to the yahoo group after the transfer will not be moved over. It's a one-shot deal, unless you ask Mark again later to transfer the straggling conversations.?
That's what I did, and it was very clean and unconfusing.?