Originally Posted by Joykins
IIRC when Tor went no-DRM, Amazon had a problem changing over, and the 1st book supposed to be released without DRM (Scalzi's _Redshirts_) actually had it on. Tor immediately sent everyone with a receipt a DRM-free book by email, if we complained, and Amazon did eventually replace the file with a DRM-free one.
That's a different case. The publisher decides whether or not to put DRM on the book when the book is uploaded
If a publisher decides to put DRM on a book initially, I believe the only way to change that (remove the DRM) is to re-publish the book.