Did this for six of Stew's posts re HumsFac site. Two stages involved: 1. Change post_main_cat_id in the posts table to the cat id for the category in the new blog. 2. Change cat id in the postcats table (there's a redundant entry there, and that's what's used by the blog machinery to decide what to show). This really should have a GUI. It's messy and we'll probably do it often enough to make it worthwhile.