On the old machine/installation, bring up an Administrator Command Prompt.

Enter the following to remove the product key:

slmgr.vbs -upk

From the new machine, go into Administrator Command Prompt.
Enter the following to activate the new machine by phone:

slui.exe 4

If you have already tried the Product key and got the message stating that the key can only be used once, you will be presented with a Windows with a phone number (855 toll free number), and a long string of numbers.

If you did not enter the product key, you will be prompted to Activate with a new key. I have not tested this, but I believe that you could enter your Product key. You should get an error. I would close the window, and try a “slui.exe 4” again. I would think that would get you to the same screen.

Call the phone number.

You will be prompted to enter the long string of numbers.

Then, you will be asked for more information. I was asked how many computers the key has been used on. I answered one (the truth). I was then given a long string of numbers that I entered in the fields at the bottom of the window.

Once complete, I clicked Activate. My installation was successfully activated.