That's a pretty big change too then! How did Lewis learn of Peter Pevensie, and how did he ever manage to turn his story into a Christian allegory?
Oddly enough, this is basically a subplot in The Magicians.
For anything in the Harry Potter 'verse to make sense, we have to assume that some (most!) Muggle fantasy is a distorted account of the actual magical realm. Otherwise we're in Discworld territory, with the causality flowing in the other direction.
In the case of Narnia, the Christian allegory probably was the distortion. The Last Battle would have to have been completely made up, anyway.
Update: Discussion has moved on to a new thread.
After 61 chapters of Harry Potter and the Methods of Rationality and 5 discussion threads with over 500 comments each, HPMOR discussion has graduated from the main page and moved into the Less Wrong discussion section (which seems like a more appropriate location). You can post all of your insights, speculation, and, well, discussion about Eliezer Yudkowsky's Harry Potter fanfic here.
Previous threads are available under the harry_potter tag on the main page (or: one, two, three, four, five); this and future threads will be found under the discussion section tag (since there is a separate tag system for the discussion section). See also the author page for (almost) all things HPMOR, and AdeleneDawner's Author's Notes archive for one thing that the author page is missing.
As a reminder, it's useful to indicate at the start of your comment which chapter you are commenting on. Time passes but your comment stays the same.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: