Thanks everyone, there are some really useful responses here.
@SMax, thanks for your guidelines but it seems that the approach you describe would only migrate the membership list. We have 10 years of history of some of these groups (going back to when they were on Yahoo! Groups) so we would really like to migrate the archives as well. If we can't do that, then we would prefer to stick with the status quo.
Luckily we haven't really used calendars, wikis etc so we don't have to worry too much about broken links.
I'll contact support and see what they recommend.
Thanks again, everyone!
Brendan.