Moderator ability to change user post to anonymous?

Hopefully, this isn’t too far off topic or too ignorant a question. What if you get a request (as an admin or moderator) to remove all data (such as a GDPR request) related to a specific user or IP address and that individual has hundreds of posts or more. Do those posts become anonymized or disappear altogether? What about replies to those posts that contain the requesting user’s name without the @?