How will it break the licence if you're installing the OS onto a single machine? If you'll be upgrading it to Windows 8 then you'll still have your licence to use Windows 7 and use Windows 8.
The Windows 8 upgrade EULA does not say "your old CD key will no longer be usable" does it? If it did that's fair enough but if it doesn't then how will this break the EULA? You're still using it as an upgrade disc from a previous installation of Windows, you're also still installing Windows 7 on the same PC, how does this break the EULA? You can't install multiple copies on different PCs that's all.
Edit: Or if you're being too pedantic then do it this way:
- Install Windows 7, upgrade to Windows 8
- Install Windows 7
There, you don't have Windows 7 on the machine more than once at any one time.