How to promote a 2003 64-bit R2 to domain controller in a non-R2 domain

In one of life’s “chicken and egg” situations I recently encountered the issue of promoting a Windows Server 2003 64-bit R2 server to be a Domain Controller in an environment with only Windows Server 2003 32-bit (non-R2) servers as existing Domain Controllers.

Running DCPromo on the 64-bit R2 will tell you that the Forest has not been prepared for the version of Windows you are trying to promote.
This would normally not be a problem, since the R2 media contains an adprep.exe to update the schema for R2 Domain Controllers.

Of course if you take your new 64-bit R2 disc 2 media and try to run adprep on the 64-bit server you will receive this error:

C:\WINDOWS\CMPNENTS\R2\ADPREP>adprep /forestprep
Adprep cannot run on this platform because it is not a domain controller.
[Status/Consequence]
Adprep stopped without making any changes.
[User Action]
Run Adprep on a domain controller.Makes sense, so of course you put the disc into your existing Domain Controller and run adprep again.

Z:\>adprep /forestprep
The image file Z:\ADPREP.EXE is valid, but is for a machine type other than the
current machine.Thats right, you can’t use your 64-bit media to run adprep on a 32-bit Domain Controller. This is where the chicken and egg scenario kicks in – how do you get your 64-bit R2 server to be a Domain Controller with no pre-existing 64-bit or R2 Domain Controllers available for the required schema update? Its not like Microsoft ships you a 32-bit copy of the OS when you buy the 64-bit edition.

The solution is to get your hands on the 32-bit R2 media. You only need disc 2 which contains the R2 adprep.exe and schema files.
Or - from Microsoft - get this hotfix that contains only the R2 adprep files.