Why should you convert it to a blog. I don't know about phpbb, maybe there is a bridge to link to a blog software to have both users synchronised.
My forum is one year and a half old, so starting a new blog would not be beneficial, since all the threads, users and posts would be lost. Why I want to convert it? Because a blog should be more interesting and usefull, could be more optimized for what I would like it to be...