I wanted to share this info for other authors publishing with Amazon KDP.
This is my first time trying to inform buyers of a book update. I searched the community boards and discovered that I had to email firstname.lastname@example.org and make a request. I received this email in return:
We received your request to provide updated content to customers who purchased your book. In order to accurately describe the new updates, we need you to provide some more information.
Please reply and provide us with details and specific examples of the corrections made for this version of your book. The more details that you provide, the better we will be able to accurately categorize and message your update for end customers. Once we receive these details, we will perform a review of your book and take appropriate action within four weeks. The possible results of our review are listed below.
1. If the changes made to your content are considered critical, we’ll send an email to all customers who own the book to notify them of the update and improvements made. These customers will be able to choose to opt in to receive the update through the Manage Your Kindle page on Amazon.com. www.amazon.com/gp/digital/fiona/manage
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 activate their ability to update the 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 found so you can fix them. Once the improvements are made, just let us know and we’ll then email customers as in case 1.
Once our review is complete, we’ll email you to share the results and action taken.
Thanks for using Amazon KDP.
I emailed them an example for each of the changes I had made: Formatting, Spelling, and Plot Changes, and am now waiting to see what happens next!