Hi,
Has the product provide a second activation?
An additional activation is provided so that you can remove and install the product on the same computer or on a different computer. See the Microsoft Software License Terms for more information. The Microsoft Software License Terms are located in the game folder.
If there is no second activation, run Flight Simulator X on two or more computers, you need to purchase additional copies of the product.
Since it's already registered on 1 machine I have my doubts that you will be able to register it on a second machine.
You may need to uninstall from the previous computer, and then activate it on a new one.
For your reference:
https://support.microsoft.com/en-us/help/927007/frequently-asked-questions-about-how-to-activate-microsoft-games-for-w
Hope above information can help you.
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