Is it possible to add a "category" to the metadata of a published article?

We are in the final stages before going live with the latest version of OJS which now includes the option to identify categories and include these in the metadata of articles. However, I am not sure how to retrofit this option for articles that have already been published. I realize that you are anxious to preserve the integrity of “versions” and therefore seem to have locked down published articles but making minor changes to indexing metadata seems a bit harsh. I must be missing something.
Janet Joyce
Equinox Publishing