First I started using categories as tags. Then got too many categories to be manageable. Then I converted them all to tags using converter from WP, which fucked up the categories. Then I got all those back to categories, which fucked up the tags.
I then tried to find a plugin that will help me deal with that mess and found none. There were few for older versions, but the ever changing WP internal structure that got more and more complex with each version forced those developers to give up.
So I did it manually. Deleted everything I did not want as category and thus ended up with 100+ posts in General category. Then one by one I assigned them to appropriate categories. Sucks, but it was the only thing that worked.
Does it really must be like this?
I did try to do it directly in the database. No way in hell. Posts are posts but are also pages. And maybe one more thing, not sure. Categories are not property of the posts or vice versa - instead they form a taxonomy. Which can be multiples for one cat/post pair. Each update is new taxonomy, you see. Really useful. And taxonomy is then one part of relationships table. The other part is object_id. Quite explanatory. You see object_id is …
Bah. I probably got half of the previous paragraph wrong. I can understand why no one wants to write a plugin that deals with that mess. I do hope Automattic has some sort of database re-factoring planned along the way. Otherwise this thing will self implode.