Typically with OEM you can change the HDD, ram and video card. Once you change the MB you get into trouble.
Its pretty much all that, but if your mobo has died, MS will still activate, but u need to do that over the phone.
Also you NEED to switch the MB before you install otherwise windows will not install the proper chipset drivers and it is possible that the machine will not boot or crash often.
True and best thing to do is a clean install.
U cannot just swap HDD from 1 PC and put it to another one. In some cases it will work fine, if CPU is similar and it has fairly new OS, not a decade old XP.
It worked when I tried SSD with W7 from C2D E6750 to 2600K, from Gigabyte to Asus mobo and everything was working fine. No errors.