I don't know how B&N handles updates to existing books, but Amazon Kindle Direct Publishing (KDP) has
documentation on how updates are to be managed.
Quote:
1. If the changes made to your content are considered major, we’ll send an email to all customers who own the book to notify them that the update exists. These customers can choose to receive the update through their Manage Your Kindle page on Amazon.com.
2. If the changes made to your content are considered minor, we won’t be able to notify all customers by email, but we will enable them to update the book’s content through the Manage Your Kindle page on Amazon.com.
3. If the changes made to your content have caused unexpected critical issues with the book content, we’ll temporarily remove your book from sale. We’ll notify you of the issues we found so that you can fix them. Once the improvements are made, just let us know and we’ll email customers just as in case 1.
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So it seems that Amazon has at least considered the impact of updates on the user.