Sounds more like the regular scenario where MBNA have set up a new Diamond Club account for the OP. The ICC if they are looking won't see miles in the member account, but won't be looking for the new account.
bananacake - I wouldn't bother with trying to get written confirmation from MBNA of the miles being transferred. The easiest way to sort this is to send a PM to Joanna at Diamond Club. She is on FT as diamond club
Send her your account DC number and the relevant background information and she will look into it for you and put it right. Give her some time though as she covers a lot of stuff for DC members. She should be able to resolve it even if it an issue with the new DC software rather than MBNA setting up a new account.