The initial WordPress installation includes only one special posting category, “Uncategorized”, which is automatically assigned to every post, unless some other category is specified. I’m adding “WordPress” as a new category, and assigning my initial postings to this category. (When I switch to posting on another topic, I’ll create another category to cover it, etc.)
Reassigning multiple previously “Uncategorized” posts to a new category is rather tedious, because there isn’t a bulk operation that handles un-assignment from a category. For that matter, it’s not entirely convenient or obvious how to do a bulk category assignment. In the Posts screen, the first step is to place a check-mark in the leftmost column on each posting of interest. Then select Edit from the drop-down list entitled “Bulk Actions”. Then Click “Apply”. This brings up a bulk edit section in the Posts page, which includes a list of category check-boxes.
Unfortunately, while categories can be assigned to multiple posts through the bulk edit interface, it does not support a bulk un-assign or re-assign capability. The upshot of this is that I might as well go through postings manually, one at a time, and use the “Quick Edit” command against each individual posting. This method does support both checking and un-checking category assignments properly, and the lack of a bulk reassignment operation is no big deal since I’ve got only a handful of posts to process. The lesson from this, though, is that it would be best to select categories carefully, in advance of making any great number of postings.