AFAIK, All pc's should be bought with their individual OEM license for the lastest version of Windows and the CoA for that license should be physically on the machine when you buy it, anything is sounds very dodgy (unless you're buying a Mac or want a Linux box).
For Windows 7 you have two options. It appears, from @chazzy2501's link, that if the machines have a Win8 Pro OEM CoA on them then you can install Windows 7 Pro (not sure how license keys work/are managed for this, I've never done it this way).
The other option is to have *any* version of Windows 8 (we normally go for Home Premium here) licensed with the PC and then have a Microsoft Volume License Agreement (Select, EES, etc) and purchase "Windows 8 Pro Upgrade" licenses, you can then use these "upgrade" licenses to downgrade to any previous equivilant version of Windows (7 Pro, Vista Business, XP Pro). This is how most schools do it.
EES saves you a bundle - both money and headaches - but you still need the OEM CoA stickers for a version of Windows on the PC's.
it says you can use VL media and key to exercise your right to downgrade OR the OEM will supply the media and key. It would appear to me that it doesn't matter how you get the software on the new laptops or what key you use as you are entitled to do so.
ooops, found this nugget: •If an end user chooses to downgrade to an approved, prior version of the Windows desktop operating system, the PC manufacturer still must apply the desktop operating system Certificate of Authenticity (COA) to the PC.
I've seen some with FreeDOS installed instead of Windows, but obviously this would only be an option if you plan on installing Linux on them.
Originally Posted by nathan
Volume Windows licenses (like the ones you get through EES) require a base Windows OS because they are only upgrade licenses.
Sometimes, the COA is under the battery on HP laptops.
it's very much possible that the Win 8 product key is embedded in the BIOS with no physical COA attached, i believe some of my colleagues have mentioned this in the past and a quick Google throws this up - HP Windows 8 downgrade to Win 7 HP 8300 SFF - No Win 7 OEM s... - HP Support Forum - 2543241
Regarding the key to downgrade, there is no magic key really for an OEM to use to downgrade a system that works forever so essentially, what will happen is that after so long the key you use will cease to function due to hardware changes and will require phone activation where they will ask for your valid Windows 8 (or key for eligible edition to downgrade).
Unfortunately this is one of the downsides of downgrading as it's a massive PITA (and i don't mean the bread).
As mentioned you can downgrade, the question will be as to what License Key you have been given and where that has come from. With the Agreement Number your software reseller maybe able to see what it is you are being given and advise from there. You would need that number if you were to contact the MS Activation Centre anyway so that would be the first port of call.