How to url re-write

[C0D3D]

I'm thinking about re-locating my forum to: forum.sitename.com instead of www.sitename.com/forum, but how can I do it so that I don't lose the database that's already in www.sitename.com/forum to forum.sitename.com? I don't want to lose all member counts/threads/posts at all.

I hope there is a way, if not I may have to resort to manual user registration/manual transfer threads and posts onto the new location x.x

Thanks in-advance.