That makes sense, since changing an operating system shouldn’t void the hardware warranty. One can always go back to Windows 8 and have all of the hardware and drivers that won’t play nicely with Windows 7 work perfectly. But an HP representative called our attention to their Windows 8 FAQ, where this actually is a frequently-asked question:
Customers who are able to and choose to downgrade their HP Windows-8 compatible products to Windows 7 will remain protected by HP product warranties. However, HP has not tested all Windows 8 platforms for Windows 7 and we may not have your particular drivers available. If you choose to install a different operating system (OS) and have a problem that requires HP support, you may need to restore the original OS to fully diagnose your problem. Please check with HP Customer Care for questions related to your specific product.
So the actual answer is that yes, you can downgrade to Windows 7 without voiding anything. That doesn’t mean that you should, though.