It is all true if you have an oem version of windows meaning you bought a pre built system with windows 8 already setup and installed to a hard drive in the system such as dell, or HP do.
But if you built the system yourself, then bought windows yourself at a later date from the likes of PC world or a computer shop or any other on line service that sells software, an OS`s.
You would of got either a link to download an iso version of windows to burn it to a recordable DVD disk.
Or The physical windows install disk in a sleeve or cd/dvd case with its activation key.
If of the later two options of how you obtained windows 8 is true then you should of got the full retail version and not an OEM version of windows.
That means because it is a retail version, you can get away with installing a new motherboard and a cpu.
And windows will still activate it`s self if using the licence key provided.
Oem versions of the os are only provided to system vendors such as Dell or HP for a pre built system with windows already installed to a hard drive of the system, on a ready to run no hassle setup on dell or hp pre built systems.
If you bought a windows disk and installed windows 8 as your first OS install to that computer, with the software bought from a shop you have a retail version of the OS meaning you don`t have to bin it.
All that will happen if you use the Licence key on another machine is it will invalidate the licence of the first machine.
And will not invalidate the licence key of windows 8 if you have just upgraded the motherboard in your old system along with a new cpu ect.