Here's the data I had to preserve when testing a mailman migration from one server to another on Ubuntu 14.04:
sudo
$ sudo su -l root
cd to mailman directory
$ cd /usr/lib/mailman/bin
get mailman list names (we're interested in wort and wort-off-topic)
$ ./list_lists
save lists configuration
$ ./config_list -o /tmp/wort.conf wort
$ ./config_list -o /tmp/wort-off-topic.conf wort-off-topic
save lists of members (both regular and digest)
$ ./list_members -o /tmp/wort-regular.txt -r wort
$ ./list_members -o /tmp/wort-digest.txt -d wort
$ ./list_members -o /tmp/wort-off-topic-regular.txt -r wort-off-topic
$ ./list_members -o /tmp/wort-off-topic-digest.txt -d wort-off-topic
cd to mailman archive directory
$ cd /var/lib/mailman/archives/private
tar mailman archive files
$ tar czf /tmp/wort-archive.tgz wort
$ tar czf /tmp/wort-mbox-archive.tgz wort.mbox
$ tar czf /tmp/wort-off-topic-archive.tgz wort-off-topic
$ tar czf /tmp/wort-off-topic-mbox-archive.tgz wort-off-topic.mbox