GUIDs are not URLs, they are unique identifiers that happen to look like URLs. It was, in my opinion, a poor design choice to make them URLs, because it causes a lot of confusion.
They are essentially a way to establish the “global uniqueness” of a post, and you should not change these for content that has already been published.
The GUID is not involved in generating permalinks or resolving incoming requests, so nothing will be broken as long as the API is used correctly, however, I have seen plugin and theme authors use GUIDs as though they were URLs, so watch out for third-party code.