I've seen "migration" attempts where one forum was left frozen as an archive. It's horrible.
First of all, the huge number of posts in an existing forum should be taken as MORE of an incentive to move that data into the new forum. The more posts you have, the greater the value of that data set. If you had only a few hundred posts then it wouldn't be a big deal when you delete the old forum.
Second, you'll eventually have to delete the old forum and that will cause major havoc. Since the old forum with its wealth of knowledge will be there, people in the new forum will be crosslinking to it like crazy. "Oh yeah your problem is just like this thread, check it out." In the meantime, the old forum become more and more dated, possibly exposing security issues. You might be forced to shut down the forum entirely...causing big problems in the new forum because all these links point nowhere.
The existence of two forums will be very confusing to new users. Anyone stumbling into the old forum from a search result will be struck by the tackiness of having a giant banner at the top directing them to the new forum. It the kind of stunt people don't do anymore, really.
I really hope the old forum data can be migrated into the new forum somehow...even an Archive section is much better than the alternative. Of course, when you choose another forum system in a couple years, you start having archives within archives...refactoring the old topics into the new structure is a better overall solution. And yeah, it will take more work than just putting an empty forum up.
I have zero time to contribute to migration beyond my above warnings from past experience, but I hope you guys can figure it out somehow.