The process was pretty simple, if somewhat painful. I moved every single post by hand (keeping the correct time-order). I did a search first – I couldn´t find a single blogsite that provided a way to import a batch of posts and still keep the old post dates, comments, and so on.
I was able to get all my old posts copied to the new site within one month, so now they all show up archived together in December 2007. Then I added a text note to the top of the archive element to (hopefully) avoid confusion. Of course, it looks like I did an awful lot of travel in December – a full year of posts all have December dates. But, where it made sense, I tried to put the original post date in the title of the moved post.
I didn´t try too hard to copy comments over – that would really have looked strange (like I was the only person to comment on my own posts – so, no)
Then I put a link from my old blogsite to the new one. I kept about a month of overlap, where I posted the same to both sites.
(of course, the first step was to create a new blog site, but blogger makes that very easy – just follow the directions)