I learned about this change a few days ago and confirmed that it is the same for Amazon Canada. I found a great video that gives really instructions on how to bulk download your books. I have 5500 books in my library. I started yesterday morning and completed the download today.
The video I followed:
https://youtu.be/A4nRGFUClr8?si=btRaJ5X8nZt2migY
There is a new video with updated scripts:
https://youtu.be/A4nRGFUClr8?si=LMIk0yWWrI48yV0i.
Good luck!