E-mail I just received from Amazon. I can't believe how exactly right this this, and how unexpected that is.
(full disclosure, I am an Amazon employee, not affiliated with the MP3 or cloud drive team. I was just independently motivated to share this, because not one hour earlier I had been looking at my downgrade options on my phone).
"Information About Your Cloud Drive Account
Hello,
Thanks for your prior purchase of the 100 GB Amazon Cloud Drive storage plan. Beginning today, all paid Cloud Drive storage plans include unlimited space for MP3 and AAC (.m4a) music files at no extra charge for a limited time. Learn more here:
Because your current plan now includes unlimited space for music, we're refunding the difference between the cost of your original Cloud Drive plan of 100 GB and the cost of a current 20 GB plan ($20), which is the least-expensive Cloud Drive plan that includes unlimited space for music. A refund of $80 will be issued to the card originally used for your Amazon Cloud Drive storage plan. Refunds are typically completed within 10 business days and will appear as a credit on your credit card statement.