Not if you make a proper Image of your 8.1 installation, before upgrading to Windows 10.
If windows 10 doesn't suit you, you can restore with the 8.1 image.
Your license will remain valid as far as this thread on the official Microsoft support forums. The person who said it doesn't represent Microsoft, though is trusted by Microsoft. (Source: Microsoft forums MVP *not Microsoft staff*)
I initially overlooked the posters credentials and assumed that he was Microsoft staff. Sorry if I confused anyone.
Computer Type PC/Desktop OS Kernel 4.x CPU i5 3570K Motherboard P8Z77-V LK Memory G.skill Ripjaw Z 2133MHz 9-11-10-28 Graphics Card GTX770 4GB Dual BIOS Sound Card Audigy 4 Pro Monitor(s) Displays 32" SAMSUNG HDTV Screen Resolution 1920x1080 progressive
Keyboard Logitech G510 Mouse Logitech G5 PSU Corsair HX750 Case Corsair R400 Cooling Corsair H100 Hard Drives 10TB total
3 RAID arrays
3 single disks Internet Speed ~900mbps (~115MB/s) down, ~10mbps(~1.5MB/s) up Browser Firefox & Chromium Antivirus Common Sense
There's a LOT more discussion of this situation on the Windows 10 forums -- and essentially, when you upgrade Win8.1 to Win10, it uses that license for the upgrade -- which means that you can't legally continue to use that license for Win8.1. You should really go to the Win10 forums to follow the discussions there: Windows 10 Forums