Email Larian (email@example.com) with the SecuROM key for the PC retail version of D2:ED or DKS to get a voucher for D2:DC from the Larian Vault
. This can also be done with some digital distribution versions, like GamersGate.
Not all retail releases had a registration key. In this case you can provide a personalized picture of the back of your box (with the barcode clearly visible) and some form of personal reference on the picture. Contact support if you have any questions.
Recently (Jan 3rd 2013), an Amazon rep in the Cheapassgamer forum said they would contact Focus about getting the Developer's Cut version (see here
). In the meantime, contact Larian support for any inquires about DRM free digital distributions.