Basically you can start from scratch with the plain old simple unmodded, subsilver phpBB that you had when you originally installed but still keep your user lists and posts, and forums, etc. All you need to do is download phpbb again, ftp in your server, go into your phpbb root directory, delete everything except config.php and upload the new files. If you dont want your board to be down, then just upload the new files into the directory on your server and overwrite everything EXCEPT config.php (that is where the board recalls the information from the database such as your posts, memberlist, etc). When your done with all the file uploads, it will look plain as hell. Any mods you had installed will not show up unless you reinstall that mod. Only then will the old settings of that mod be recalled from the database it was stored in and work again. So technically, the mods will still be there but not be used unless you tell it to.