Sorry about the late reply.
Since we'll need to migrate our old mailing list archives to this new infrastructure (and preferably keep the URLs stable), the migration may be quite a bit of work to prepare. So, I'm looking for people who can help out: find out which *exact* steps we have to go through to get minimal downtime on an upgrade and get back to a stably running system on return.
The mailman archives are currently split up by month.
If the same is true with MailMan 3 as well, how about creating/importing the mailing lists as a parallel structure in MailMan 3 and, with a month switchover, switch the postfix routing to MailMan 3?
As a second (independent) step we can Redirect the HTTP URLs for new archives to the new (parallel) location. (And find some way to merge the new and old Archive pages, but that might "just" be a perl script).
I'm proposing that because I've known someone who did a mailman switchover in the past (though not 2=>3), and it was a real PITA to keep the URLs stable. (Not sure whether they ended up with a big Rewrite table?!).
This here sounds like a least-risk solution -- we can even have postfix deliver mail to _both_ mailman installations at the same time, to be able to switch back as a fallback path.