Wordpress migration

I have a Wordpress blog with about 200 posts on it. I need them moved to another URL and a custom 303 redirect setting up for every post. Comments also need migrating and entries editing to point to pictures on the new URL rather than the old.

Note that you can\'t just do an unload/reload because
1. Extract file is > 2MB
2. I need only the posts and comments NOT the pages.

Permalinks on new site are different and so are categories (I can provide a mapping) hence the individual 303s.

Both sites are live and it\'s imperative not to lose Google ranking.