The only thing you can save is your data and personal things like spreadsheets, pictures, music, etc. Windows, along with any programs you have installed, will have to be reinstalled from scratch. Its a hardware issue, not so much license string. You current installation of Windows has all the drivers and configuration set for your current motherboard. As stated, unless the new motherboard is identical, or very close to what you have now, Windows will not have the configuration it needs to work with the new board. There are ways to get around this, doing repair installs on the new board sometimes works. But, people who do this all the time will tell you that if its a different motherboard, your fastest and best bet is to do the new install. Its just not worth taking the time to sort through all the problems of trying to get the OS to work doing a board change.
If the motherboard is close, and you can try, Windows may boot and reconfigure its self to the new board. But be prepared by saving everything important to another drive first before you try.