Recent versions of mailman strip DKIM signatures, rewrite the envelope-from to use an address at the list's domain and set reply-to to the original authors address to resolve the DMARC issue. I'm on several lists that do this and it works just fine.
+1 on moving the list. Given the fact that the mails are archived in public, it's not really a huge deal how it takes place. One month sounds reasonable (although I think it could be done on a shorter timescale). I'd setup the new list to allow subscriptions, but keep it moderated to keep discussion from moving until the cut, send lots of warnings and then on the big day unmoderate one and moderate the other.
It's a great opportunity to hardfork something in Bitcoin without risk of breakage, losses or entertaining melodrama. : )
--adam
ps. I think SF will let project admins download mbox archives of the list, the new admins should be able to import them to keep archive consistency in one place.