How to set categories of post through custom fields?

Hey WP Freaks,

I have a custom meta box which allows users to select from a set of categories in a drop down list.
To use this selection as the category of the post I have then successfully used wp_set_post_categories function in a custom save_post action.

HOWEVER this doesnt work when I want to edit the post after submitting though. Ive been at this for a week now, any ideas why this is not working or for alternative approaches? How does one do this through a wpdb query for example?

Thanks,

Balthazar