I'm reluctantly posting the attached file.
Its from the conversions discussions before - but I've butchered it a bit because I wasn't getting my threads over as threads...
I would not call this a 'solution' its a try it and see...
Its not pretty. The code isn't mostly mine (nor is it credited properly... but i think its from the SB convertor originally). If ur forum is big it may well die.
If you haven't synced your users already then this might do it but I commented that code out cause mine were synced.
BACKUP
AND RUN ON A TEST SERVER FIRST!!
it runs on the assumption the two forums are in one db, if not copy them over first.
I put these files into ../components/smf2fb and then accessed
www.mysite.com/components/smf2fb (I have SEF enabled) otherwise access
www.mysite.com/components/smf2fb/index.php
You need to add username etc to the mysql file.