DISM to upgrade Dell Windows 2008R2 Standard to Open License 2008R2 Enterprise

My client requested to upgrade from Windows Server 2008R2 Standard to Enterprise in order to utilize memory above
32GB.  The server is a Dell T420 purchased with Windows 2012R2 and Windows 2008R2 installed by client request.

Per recommendation of the "IngramMicro Microsoft Licensing Desk" I purchased 2 copies of "Windows Server 2012R2 Standard"
"Microsoft Open License" and received keys for Windows Server 2008 and 2008R2 Std/Ent as well as the two 2012/2012R2

I received this key:
Windows Server 2008R2 Std/Ent - MAK B MYKEY-MYKEY-MYKEY-MYKEY-MYKEY MAK 0/45

To perform the upgrade I followed the instructions found here:

To determine the installed edition, run:
DISM /online /Get-CurrentEdition
To check the possible target editions, run:
DISM /online /Get-TargetEditions
Finally, to initiate an upgrade, run:
DISM /online /Set-Edition:ServerEnterprise /ProductKey:MYKEY-MYKEY-MYKEY-MYKEY-MYKEY

The first two steps completed with affirmation that ServerEntrise was an eligible target from ServerStandard.

The third step errors with Error 1605 and from the log:
2015-09-11 22:48:54, Error                 DISM   DISM Transmog Provider: PID=5960 Product key is keyed to [], but
user requested transmog to [ServerEnterprise] - CTransmogManager::ValidateTransmogrify
2015-09-11 22:48:54, Error                 DISM   DISM Transmog Provider: PID=5960 [Upgrading system]: The specified
product key is not valid for the target edition.
Run this command again with a product key specific to the target edition.
 [hrError=0x80070645] - CTransmogManager::EventError
2015-09-11 22:48:54, Warning               DISM   DISM Transmog Provider: PID=5960 The selected OS *cannot* upgrade
to [ServerEnterprise] - CTransmogManager::TransmogrifyWorker
2015-09-11 22:48:54, Error                 DISM   DISM Transmog Provider: PID=5960 Failed to Upgrade! -
2015-09-11 22:48:54, Error                 DISM   DISM Transmog Provider: PID=5960 Failed to upgrade! -

Noting that the first error contains: "Product key is keyed to []" I searched and found:

Assuming this article puts me on the right track, should I 1) "change" to Generic "Standard" Key then Upgrade to my own "Enterprise"?
or 2) upgrade with Generic "Enterprise" Key then "change" to my own?

GENERIC KEYS FROM: https://technet.microsoft.com/en-us/library/ff793421.aspx
Generic W2K8R2 Standard KMS Key from Microsoft:  YC6KT-GKW9T-YTKYR-T4X34-R7VHCv
Generic W2K8R2 Enterprise KMS Key from Microsoft:  489J6-VHDMP-X63PK-3K798-CPX3Y

Using Generic KMS Keys:
DISM /online /Set-Edition:ServerStandard /ProductKey:YC6KT-GKW9T-YTKYR-T4X34-R7VHCv
DISM /online /Set-Edition:ServerEnterprise /ProductKey:489J6-VHDMP-X63PK-3K798-CPX3Y

Our Key:
DISM /online /Set-Edition:ServerEnterprise /ProductKey:MYKEY-MYKEY-MYKEY-MYKEY-MYKEY

Also, should any of the upgrade/change be performed with: slmgr.vbs /ipk <MultipleActivationKey>
as from this Technet Article: https://technet.microsoft.com/en-us/library/ff793408.aspx

Other Suggestions or procedures?

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RantCanSr. Systems AdministratorCommented:
Use the Generic Enterprise KMS Key to get your upgrade finished and then install/activate your actual key afterward.

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