Isolation of Certain MySql tables to allow me to start again

Currently I have a Phpbb forum, which I intend in performing a large upgrade to.

However, due to third party modifications and various other additions to the forum, it has become very difficult for me to isolate the require tables to allow me to start again without losing certain data.

I wish to literally install a new phpbb forum, from scratch, which keeps the current Posts, Users and Private messages from the old forum.

Currently, I cannot simply take those specific tables and put them into a new forum, as I always seem to encounter an error, such as the requirement of the old forum style, when I just want to go to default.

In my opinion, nothing else is required, I just want the posts, users and private messages.

Once I have a default forum with no modifications and all the posts, users and private messages from the older forum. I will be happy.

If other tables are required for this to actually be possible, by all means include them, all I want at the end of the day is a nice fresh forum, with my posts, users and private messages intact. No modifications, just brand new with my data.