Microsoft recently made a change to the licence agreement saying that a new motherboard is equal to a new computer, hence you need to purchase a new Windows licence. Here is what Microsoft has to say:
“An upgrade of the motherboard is considered to result in a “new personal computer” to which Microsoft® OEM operating system software cannot be transferred from another computer. If the motherboard is upgraded or replaced for reasons other than a defect, then a new computer has been created and the license of new operating system software is required.”
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