I won't use both, only the new one (old parts will be given to friends).
So I could uninstall my 7 key on my current PC, install 7 on the new PC and then upgrade it to 10?
Sorry in advance if I'm being thick...
In my understanding, here's how it works:
- Case A: OEM.
If you have an OEM version of windows 7/8/8.1, your license is bound to specific hardware (basically the motherboard). So if you download Windows 10 before July 29th 2016, you'll be able to re-download, downgrade, re-upgrade at anytime on that exact same hardware during its lifetime.
However, your Windows 7/8/8.1 license is OEM and therefore not transferable to begin with. So if you move to new hardware, you won't be able to use it legally at all. As a consequence, you won't of course be able to upgrade to Windows 10 with your key either, even if you attempt the upgrade before July 29th.
In order to install Windows 10 on a new build, you'll either need to purchase a new valid license for Windows 7/8/8.1 which you could then upgrade for free before July 29th, or you'll have to purchase a full Windows 10 license.
- Case B: retail.
If you have a retail version of Windows 7/8/8.1, the license is not bound to specific hardware, and therefore, transferable to a new machine. So even if you have already upgraded with a specific machine, you'll be able to transfer your Windows 7/8/8.1 license to a new build and then upgrade to 10 for free provided you do it before July 29th. After that date, you'll need to purchase an upgrade license.
(Edit: this little trick here needs clarification: even if you have performed the upgrade with a specific retail license before July 29th, the Windows 10 license that's granted to you is basically considered OEM. So if you move to a new build after the deadline, you will legally be able to use your retail 7/8/8.1 license on that computer, or even the next one, but you won't be able to upgrade to 10 for free anymore.)
It seems that you fall into case A, so basically, no luck.
(Unless some of the hardware you purchase for your new build comes with an OEM license for 7/8/8.1, and you perform the upgrade before July 29th.)