Changing the odd word here and there must be fine , mustn’t it , because some words are now rightly totally unacceptable.
No. You shouldn't rewrite books because in doing so you're rewriting history. Books can be allowed to fall into obscurity, to be of interest only to academics or historians. But to rewrite books is to obscure their context and place in history. It's just not okay.
Agatha Christie is one example. But another example would be Mark Twain. His books for children use the N-word extensively. Because he is writing about racism. Same as To Kill a Mockingbird. If you take the racial epithets out of that book, you are not demonstrating what the problem was they were fighting against. You can't have an effective fictional book about racism without showing racism.
When I was a kid, I read all sorts of books with all sorts of words in them that were "totally unacceptable" then as they are now. But even as a child, I was able to understand the historical and cultural context and these books taught me a lot.
One question we haven't asked is, what is the purpose of removing "unacceptable" words from books? What is this achieving? It doesn't end racism, sexism, fatphobia etc (it might counterproductively cover those things up.) Coming across an "offensive" word in a book is an opportunity - to discuss, debate, deepen understanding, open a door into the past. What is the purpose of taking away this opportunity?