Board Thread:Support Requests - Getting Technical/@comment-26347760-20150621220703/@comment-3474542-20150624072129

Special Import and Export would only add categories to pages, not remove categories from pages. To answer the OP's question, unless the categories are added by a template, each page needs to be edited individually. However, these types of edits can easily be done with a bot program on a bot account, such as the one volunteered by Imamadmad.

If the category is added by a template, then you can change the category on all pages at once just by editing the appropriate template page.