As with install the conversion is automated. Your previous 2.0.x database tables will not be changed as well as the original 2.0.x files remaining unaltered. The conversion is actually only filling your phpBB3 database tables and copying additional data over to your phpBB3 installation. This has the benefit that if something goes wrong you are able to either re-run the conversion or continue a conversion, while your old board is still accessible. We really recommend you disable your old installation while converting, else you may have inconsistent data after the conversion.
Broken non-latin characters The conversion script assumes that the database encoding in the source phpBB2 matches the encoding defined in the lang_main.php file of the default language pack of the source installation. Edit that file to match the database's encoding and re-start the conversion procedure.
http 500 / white pages The conversion is a load-heavy procedure. Restrictions imposed by some server hosting providers can cause problems. The most common causes are: too low values for the php settings memory_limit and max_execution_time. Limits on the allowed CPU time are also a frequent cause for such errors, as are caps on the number of database queries allowed. If you cannot change such settings, then contact your hosting provider or run the conversion procedure on a different computer. The phpBB.com forums are also an excellent location to ask for support.
Password conversion Due to the utf-8 based handling of passwords in phpBB3, it is not always possible to transfer all passwords. For passwords "lost in translation" the easiest workaround is to use the "forgotten password" function.
Path to your former board The converter expects the relative path to your old board's files. So, -for instance - if the new board is located at http://www.yourdomain.com/forum and the phpBB3 is located at http://www.yourdomain.com/phpBB3, then the correct value would be ../forum. Note that the webserver user must be able to access the source installation's files.
Missing images If your default board language's language pack does not include all images, then some images might be missing in your installation. Always use a complete language pack as default language.
Smilies During the conversion you might see warnings about image files where the copying failed. That can happen if the old board's smilies have the same file names as those on the new board. Copy those files manually after the conversion, if you want to continue using the old smilies.
Please shout out any comments, questions, or experiences you have first or second or third hand about phpBB 3.0.