I didn't suspect that because in all my correspondence with Amazon, they always said the issue was incomplete multipart uploads.
So I assumed, based on the dialog box called "Edit storage class", that the net result of this action would be the same files but with a different storage class. This led me to read the warning as "We will create a copy of your objects and delete the old one, so please be aware you will be charged for the cost of the copy" rather than "We will create a copy of your objects and two versions of them will exist forever."
So it does seem like Amazon tried to warn people. But why on earth would a dialog box labeled "Edit storage class" not do anything of the sort? Are there actual legitimate reasons someone would want to have two versions of their objects sitting around?
Say what you will about my billing awareness (and you are saying it. Feeling the hate, all, thanks) but this is not good UX.
If there's any legitimate reason to duplicate objects in the name of changing the storage class, I'd love to hear it.