1) Win 7 / 10 will allow you to install the OS on to a new pc these days, 99% of the time.
Using the license key from the current install. I registered my last copy of Win 10 through a Microsoft account, skipping the prompt to input the license key during Windows install, then registering it with the Microsoft account.
On a future build, I can install Windows again, register that install, and make the install on the old build invalid.
One can also (usually) input the license key. This is not always true, but with a phone call to Windows support, one can usually be ok.
In years past, this was not true, but Microsoft has become much more open-minded about not making you buy so many copies of Windows.
2) When you do transfer a...