There is the “default categories watching” site setting. If you change it, you’ll be given the option to apply the changes historically. Selecting that option will update the preferences for all users except for users who have explicitly configured their preference for that category on their preferences page. I think that’s the issue you are trying to get around. You could probably deal with that from the Rails console, but I’d probably avoid that approach.
You could just create a new category for announcements and add that category to your site’s “default categories watching” site setting. You could then tidy up the site by moving topics from the old announcements category to the new one. If you want to keep the category name “announcements”, start by renaming the original “announcements” category to something else. I realize this is a bit of a hack, but it’s a way to resolve the issue without touching the Rails console.