Hmm, I like temptations to think about
I just think outloud now maybe there's some solution in it.
There was a time I worked with IB 1.3 and the crowd was happy and there was an active coding going on that version.
I did tested then a kind of 'port' function between two boards, making it so that when I put a message in a special created forum, it apears in an other connected board in the other forum. It was called warping. At the time we had two support forums and we used that to have a copy of support questions in both boards.
There also was a way to show complete topics from one board in the other.
In both cases it was needed to have both forums in the same dbase with a different prefix. The usability depends if the showed forum needs to be active or that it is about the info in it and it's not needed to change them anymore.
Now you're problem is to merge two boards, I think it's wise to splitup the problem in workable chunks.:
1. both boards need to become converted to SMF,
2. Both boards need to be merged.
I think this way it's logic as it seems more logic to combine two boards together when they have the same nature.
Then one (with some time at hand) makes a list of the hundred users that have to be saved, with the (new?) passwords to be used.
Then openup both forums, registrate the member in the board that has allready the most users, so they become added.
Then login in the old board as that member and copy the messages and place them as that user in the new board.
That can be done by hand, maybe even a little script to speed things up. It's time consuming, but if the info is real important, it's worth the work.