Just to clarify one thing again, I contacted the customer support by a ticket. I asked whether we receive a full refund after replacing the digital copy with the CE and this is the answer:
Thank you for contacting us.
After the actual release of Diablo III full refunds will likely not be available. However, if you upgrade to the collector's edition we'd likely be able to issue a replacement CD-Key for the standard copy that was on the account.
Personally though for the situation you described to avoid complications, if you want the collector's edition then just go for that instead of the pre-order.