You can re-use your Windows disk anytime, but without a license you would a) be limited to 30 dayys before you're forced to actuvate and b) have to work with greatly reduced funtions.
By using your old Windows 7 license key to "upgrade to WIndows 10 you'll basically converted it into a Windows 10 key.
To reinstall Windows 10 you would need a Windows 10 installer instead.