And welcome
So converting from phpBB2 is exactly the same using the premod or phpBB vanilla.
You need to install phpBB3 (or the premodded version) first, in a separate directory in the phpBB2 domain and in the same DB.
Doing this will not alter phpBB2 in any way as long as you use a different table prefix for phpBB3.
You'll as well be able to convert without altering the phpBB2 install once done, and see how it goes before you actually switch.
35000 post should not be this much of a deal to convert, won't be done in a minute but processing time should stay reasonable (should take less than one hour unless your host really is slow).
This means you''ll most likely will end up converting twice, the first one to see how it goes and to set up the SEO settings (url and personalised forum urls), and then, when everything works well, a final conversion to take the recently posted message in phpBB2 into account.
You should deactivate posting in phpBB2 (locking all forums for example or deactivating the board) when you go for the final conversion to make sure user will not post during the process (so you do not miss any post).
Once done, you'll have to rename phpBB2 dir (or move phpBB2 file within a dir if installed in the domain's root) and then rename phpBB3 dir to the original phpBB2 one (or move the phpBB3 files to root in the root install case) to actually switch from phpBB3 to phpBB2.
Then, you'll have to update the phpBB3 config a bit, you can either do it before you move phpBB3 files or after using phpmyadmin, you just need to set up the new script path (since the phpBB3 dir will have changed). Once done, clear phpBB3's cache.
Your SEO settings will be kept during the final conversion process, but you'll have to update the .htaccess for the new phpBB3 location, you can either edit it by hand (replacing the old path to the new one) or generate a new one once phpBB3 is up and running in the new dir with the updated script path config.
If your phpBB2 installation was not using any mod rewrite, you'll just need to activate the zero duplicate and you'll be done.
If not, you'll most likely have to implement some personalized rewriterules in order to properly redirect the old urls.
Once everything is working nicely, you can safely delete phpBB2 files and tables.
