You *may* be able to reuse a Windows license- depending on how you acquired your previous Windows license and what kind of license it is. As I understand it, OEM installations stay with the machine, but if you bought a retail copy or an upgrade license, you*might* be able to transfer it - but NOT use it on two machines simultaneously.
I think you can still get a"free" copy of Windows 10 if you participate in the Windows insider program or if you need to use"accessibility" features because of a disability...
You need a license, but you can download the installation tool from MS and use it to burn an install USB key. You can use Win 10 for 30 days without a valid license I think.
You can find license information here:
https://answers.microsoft.com/en-us/windows/forum/windows_10-windows_install/windows-10-how-to-tell-... Windows 10 - How to tell what type of Activation I have, Digital - Microsoft Community