Migrating contents and comments from an old Wordpress blog to a new, also Wordpress, blog?

A friend of mine had a Wordpress blog hosted at his ISP. That blog is actually still active.

Now he created a new blog at Wordpress.com and has made a last post on the old one linking to the new one.

But I suspect there would be a better way…

What do you suggest?

A plugin to migrate everything?

Reinstating the old blog on a new server with a decent URL?

Please let me know your thoughts, thanks