If I change the permalink (URL) of a chapter in the webbook, can users still access it with the original link?

Yes. 

If you decide to change the permalink for your chapter in the chapter editor interface, the previous URL will automatically redirect to the new URL. The reader will therefore still be able to access the chapter by using the old URL.

However, if you create a new chapter and reuse the old URL, then the old URL will only link to the newly created chapter. 

If you then delete this new chapter, the old URL will again redirect to the original chapter.