i just managed to convert Invision 2.x to MiniBB with a small re-route...
let's say, you have invision in your /board/ folder and like to convert it.
1. just install phpbb into /phpbb/ as normal and database should be the same as invision's db. notice that the suffix have to be ipb_ for invision and phpbb_ for phpbb.
2. install minibb into /minibb/ and use the same db above with the suffix minibb_
3. convert invsiion to phpbb with the converter
. Read the txt file carefully to run it correctly.
4. grab the sql script at minibb forums
and with a minor change, inject that with phpmyadmin into the only one db we're using. as all tables are on one db here, we need to remove the PHPBB. (with dot) trailer from the tablenames. Do a search/replace on your favourite editor (should be EditPlus, if not throw everything away, hehehe) and use the resulting sql.
5. delete invision and phpbb folders (so that some lamer does not abuse it to get intoyour minibb). you can also go to phpmyadmin to drop all tables beginning with phpbb_ and ipb_
Notice that to use the script easily follow these steps, otherwise you have to dig into some codes/sql's. But this method easily converted my UTF-8'ed forum very nicely without loosing anything.