If you only installed SEO mods, it's trivial, do as HB says, migrate you DB to phpBB 3.0.1, without tacking car of the seo mods (eg going back to phpBB3 original files) and once done, install the premod over it, as explained here
This way, you will not have to edit a single line of code.
Since you are already using the phpBB3 SEO Mod rewrite, do no delete your phpbb_seo/ dir, you need to keep your settings.
If you did not mod the phpbb_seo_class.php file, then overwrite it, otherwise, merge it, and keep the files in phpbb_seo/cache/ to keep you settings and configured forum urls.
When the new premod is installed, proceed as explained for updates, go acp and configure new settings before you generate a new .htaccess and voilà