Recently, I changed a monitor from 2560x1440 to 2048x1920 and the X51 updated to Windows 10. Next, without any hardware changes, the XPS 8700 updated to Windows 10.
Aurora R1 is running Windows 7.x
Aurora R3 is running WIndows 7.x
What MS OS can the Aurora R1 support (2560x1440 display, updated GPU)
What MS OS can the Aurora R3 support (2560x1440 display)
Can either move to Windows 8.1.x? Or both?
Can either or both move to Windows 10.x?
What is significantly different between Win 8.1.x Pro and Win 10 Pro?
First there is a free upgrade from Windows 7 OEM to Windows 10 Retail so there is absolutely zero point in purchasing a Windows 8.1 Retail license...
Windows 10 TH2 is essentially a fixed version of Windows 8.1 where Metro Apps work correctly as "Windows".
Almost any system shipped with Windows 7 can run Windows 10. As they are 2009 and 2011 models respectively Dell did not provide updated drivers for Windows 8.x or Windows 10 because they are all out of warranty. All the drivers should be inbuilt into Windows 10 TH2 or automatically obtained via Windows Update. Its advised to update your BIOS to the latest revision before proceeding.
A Clean Installation also in general performs better. See here for details:
Both systems likely have a legacy (non-UEFI) BIOS so take that into account when making the Bootable USB. Although you may want to look at the BIOS settings for the 2011 model to see if there is UEFI support.