I just had to do that myself, so I made a something that would convert it for me. You'd probably be better off using something you have to pay for to convert a huge forum, but I had this working for around 20,000 posts.
To use it, you are going to want to extract it, and then change that database info at the top of the file. Upload it to the server where the access database is located, and run the file in your web browser (I found it to work better in FireFox).
For the tblThreads, tblTopic, tblForum, tblAuthor, etc, you are going to be wanting to copy over the Identity field, so leave the 'Don't Copy' checkbox unchecked for the Identity fields on those, so it will copy the Identity.