Using Windows 8.1 with a system designed for Windows 7 shouldn't be a problem. Using Windows 7 with a system designed for Windows 8 or 8.1 is where you might run into trouble as there are often no Windows 7 drivers for hardware released after the release of Windows 8. Network adapters and sound hardware are often the biggest issues as Realtek and some other manufacturers don't like having to bother with supporting drivers for older systems when the hardware was designed to be used with a newer OS.
Most your Windows 7 drivers are probably natively supported in Windows 8.1 or have newer 8.1 drivers available from the specific hardware manufacturer or Samsung. It should not be a problem.
It's also noted at the bottom of the page for that...