Stupid shits at the mobile carrier. Their websute clearly says NOT to put in your account number if yours is a prepaid service. So when activating a new SIM card whilst retaining the existing phone number I left the account number area blank.
Then an email arrives. There's an error with either my birth date or the account number.
So instead of leaving it blank this time, I entered the account number successfully. Now the transfer from my current micro SIM to the new Nano-SIM can proceed.