Moving posts from version three to version four

[Moon Moose]

I'm sure there must be a thread for this, but, sorry I can't find it.

I'm changing from vb3 to vb4 and want to move the posts.
I'm building the vb4 forum on a new additional url whilst keeping the vb3 forum ticking away.
When the vb4 forum is ready I want to copy all the posts to it and repoint the old address go to the new url.

I don't know how easy or difficult moving the posts to the new frame work might be. I don't know whether it has to be tackled in a different way. Can anyone advise?