What I would suggest is the following... but please, make a local test... don't go live unless you feel confident in your skills.
- Make sure you have a working copy of your current phpBB based FORUM system.
- Remove all languages and force your users to use ENGLISH via DB (an SQL like this should work: UPDATE phpbb_users SET user_lang = 'english';).
- Remove all templates but keep subSilver with theme_id = 1 and force all users to this (an SQL like this should work: UPDATE phpbb_users SET user_style = 1;).
- Use this file to clean your tables: phpbb_clean_tables_1_0_8.zip (you will find it in the contrib folder of IP).
- Make sure you will delete all the fields and tables unless strictly needed (for example you may want to keep album and attachments... so do not flag them!).
- When you have "cleaned" your DB, your forum won't run anymore... so you should delete ALL your old files BUT the users files (album, attachments, avatars, etc...) and config.php.
- Copy default phpBB 2.0.23 files into the forum folder (all but config.php).
- Your forum should now be backported to phpBB and shoud work fine: you have now a new starting point and you can try to follow upgrade from phpBB instructions.
- After having reupdated your site, you should move users files into default IP folders so all users files will be there...
I would suggest installing IP 1.1.10.25 (or check if there is a newer version when you read this message) with all patches, because IP 1.2 will be out soon and your upgrade will be easiest if you start from this version.
Read carefully what I wrote... and let me know if you succeed.
I keep my finger crossed.
