Historically the retail channel has always had more allowances for hardware changes. Volume licence agreements and hardware changes are easily solved by talking to your MS Rep. An example is the old System Builder licence which stated that I had to supply the licence with specific hardware (basic machine). If I attempted to activate the licence on new hardware to soon it would fail. If you used the phone activation and told them that the MB was replaced due to hardware failure then it was OK. If you told them you were upgrading then they would give you a long story about the legalities of the System builder licence
I had backed up windows 8 prowmc activation using ATM 3.5 beta3 . My primary HDD went rogue so RMA'ed it and meanwhile bought a new hdd , installed windows 8 pro used kms activation and tried to use new wmc free upgrade key and failed to activate but i tried to restore the previous activation backup after installing previously used wmc key and whoa ! Windows 8 prowmc got activated . So i think u can change ur hdd in which u installed ur os .
I did change gpu, primary disk, keyboard and mouse, all at the same time on my kids computer and it didnt trigger the reactivation.