I am running a home built computer:

Main Board ECS H81H3-M4 board, LGA 1150 socket
CPU INTEL i5-4570S CPU (missing hyper-threading)
Memory ECS 16Gb (2X8GB)
Graphics GPU on-board INTEL 4600
SOUND VIA HD on-board audio (Win 10)

Windows 10 is free upgrade from retail Windows 7 OS.

Due to PSU burnout, this hardware replaced the original HP mainboard & cpu. It was activated successfully under license while W/10 was still free. I had followed rules & set up a Microsoft account before activation.

fast forward to Sept. I bought a used MSI Z97 PC MATE Socket 1150 MoBo, updated the BIOS successfully & installed it. Microsoft on-line activation would not activate Windows 10 saying no license, no Key. I followed all the procedures, including going to the store & running the "replaced hardware" troubleshooter. I understand that the license is tied to the main board somehow.

I built the system back on the ECS board & it activated.

I want to replace the Intel H81 board because it is a micro-ATX & is a very basic board. (not with the MSI board. In all my breadboarding & assembling, it went black screen, dead. Used, cheap, no loss)

How do I replace the main board & get it to activate?Read the following, it may help you out.
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-changeMicrosoft likes to associate the Motherboard/CPU to their licensing. If you go the method of talking to someone in Microsoft support they might grant you an activation. I havent tried this activation route with Windows 10, but back in the day with Windows XP I had an issue with a system that its motherboard was replaced but the CPU/RAM and all other hardware remained the same. The System Recovery CD that CAME with the computer wouldnt work with that replacement motherboard because the Compaq System Recovery CD detected that the motherboard wasnt original. I ended up using a slipstream CD of Windows XP SP2 and using the key on the label on the computer case and then RAN into activation issues to which point I had to explain my issue to Microsoft India Support. I stated that the motherboard was replaced and the guy ASKED me a few other questions to which I shared with him the make/model of the computer and that everything else is the same hardware the main board died and this replacement while the same board didnt have the Compaq BIOS Flash and so the difference caused it to not activate as the new motherboard didnt show the Compaq logo like the original board did. I was granted an activation and all set when entering the key activation info. Any issues with aciivation are solved with a 2 minute phone call...it;s Free,Thanks to everybody who answered. Term-ite; I had an microsoft account linked when the motherboard was replaced. activation said so. With the replacement mobo, that statement didn't show up anymore. I went to the store, clicked hardware replaced and used the troubleshooter. No good.
Dave Lembke; I've made one phone call to microsoft some time ago wanting to activate a computer. It was a disaster. We literally were not speaking the same language. My system is a case off the shelf with parts from NewEgg, Ebay and local suppliers. No case label, no manufacturer. Windows 7 is a legal retail copy purchased on line. Windows 10 is a free image linked to windows 7. I'll try a phone call when I get another mainboard.