Lately, I have been carrying out research on PC components, which I have a tight-ish budget of $1,650 USD to spend on. However, I was recently made aware that the GTX 1070 is a better GPU than the RX 580 (the GTX 1070 is more expensive), in addition to the Corsair LL fans doing a better job than the stock fans on the Corsair H100i. Due to the additional strain that these two components will put on my budget, I subsequently cut the (retail) copy of Windows 10 from my part list, which ultimately led me to wondering whether or not I can pull the hard drive from my current PC to use with my future one until I can wait slightly longer to purchase Windows 10. My hard drive has an OEM version of Windows 10 on it, from HP in particular.
If you're wondering, here is a spreadsheet outlining the cost of everything I will be purchasing for my new gaming PC this upcoming winter: https://docs.google.com/spreadsheets/d/1XZQJgF_k2uCS9sac3X0UZIe4kg7Rhh-x2ZI8DwXMbys/edit?usp=sharing
EDIT: Another reason I chose to not get Windows 10 was because the i7-9700K was rumoured to be released this autumn; it will most likely be more pricey than the i7-8700K.