User name appearing on posts

After seeing the number of unauthorized people attempting to login to my WordPress site, I’m now seeking to address some security issues. My first issue was to not display the username on posts. To do so, I created a Nickname and a Display name publicly as that did not match the username. When looking at existing posts, however, I am still seeing the username and not the Nickname or Display name publicly as names.

Does changing those only affect NEW posts?

If so, is there a way to change existing posts to display the sanitized values?

If not, can anyone suggest what I did wrong?

Thanks,
Todd