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1) Upgrading your Windows License is not what you are doing, since you're starting from a new mobo==new machine, so you aren't looking for an upgrade. Microsoft is not being greedy, you bought or got the wrong product initially. If you wanted a transferrable license you should've paid extra originally for that.
2) There are student pricing from microsoft. type it into google and it will bring you to microsoft's site. If you are in college or a bigger school with an actual college tech store, ask around the student computer store and they may have more discounts then publically available.
OR you can bundle another OEM version of windows again with your new build and save now. I would guess your new computer will last you longer then you will need the transferable license.