Moving a mailing list to a forum?
Don't do it.
Being able to read off-line messages, replying to them at leisure, and sending them when next connecting to the intarweb means a lot to those of your listmembers who are on a dialup connection.
Then there's the thing that goes: "I don't have time to read my lists right now, I'll just let the messages collect over there", on email messages. It's doable on a forum, too, but it's more difficult.
Then there's the thing which goes: "How absolutely fascinating! I'll have to reply to it later" (and you press "flag for reply", in your email software. Or you press "reply" and "save as draft").
And you actually remember to reply to things, instead of getting distracted ... oooh, nice kitty, oh, it's bedtime already, I'll just close the browser then. Seven tabs open? Yes yes, close the lot, thanks. Goodnight.
There's also the difference between pressing "save message" and "save this page" ... one saves a message, the other a long long thread, with lots of irrelevancies thrown in. And the thread on the page you just saved lives on, and you've missed one or the other crucial reply ... cos you saved the page already, didn't you? If you save it again you'll have two copies, similar but different.
Nah, email is niftier again.
All of which means that, if you're contemplating moving a mailing list to a forum, think again. Only about 10 %, give or take 10 %, of your mailing list members will ever bother signing up to your forum.
And of those forum members, only a fraction will ever bother posting anything at all.
There's also the initial disappointment: "There's nothing here but a 'welcome to this forum' post, and a 'here's the posting rules for the forum' ... bah humbug, I'll have a look at my mailing lists instead." They might come back for another look a week or three later, but then again, they might not.
If your forum sports all of two more posts a week later, you can close it down again - it's dead. You've seen those forums, too, with just three posts, all written years and years ago - and all by the admin.
No, stick with your mailing list, if it's thriving. If it's busy but oh so overloaded with junk (idle chatter, arguments and the like) - well, clean it up. Ditch the junk posters (or moderate them), and enforce a bit of rule on your list. It'll bounce back like there was no tomorrow.
Because the population of a mailing lists is completely different from that of a forum, and you can't force people to move on over.