Looking a bit further into the best way to do this - since mailman doesn't have any sensible export/import features that retain list member settings, we'll probably need to make a full copy of the list on the server, and then remove the members of the old one. I'll ask Daniel to look into that next week and have held off for now.
As for archives, Daniel says it shouldn't be a problem to keep the old archives under the old URL, but to also to copy them (with new URLs) into the new list. The only disadvantage I see that in the event we need to do any removals of old posts, we'll need to remember to do it in both places.
All best, Erik