1st install WAS locked to motherboard, not anymore
Reactivating Windows 10 after a hardware change
In Windows 10 (Version 1607 or later), you can link your Microsoft account to the Windows 10 digital license on your device. This can help you reactivate Windows using the Activation troubleshooter if you make a significant hardware change later, such as replacing the motherboard.
https://support.microsoft.com/en-us/help/20530/windows-10-reactivating-after-hardware-change
Anyone with a Digital entitlement can move their copy of win 10 to a new motherboard once they either logon using a MSA (Microsoft Account - which is the email address you used to register win 10) onto their current PC or go to settings/update& security/Activation and run then troubleshooter there to re activate their device after reinstall.
Now retail may be different, as far as I know you still have a digital entitlement so there should be no restriction to move it in the same way.
Microsoft seem to have blurred the lines between OEM & Retail.
You could use that USB, but I would download a new copy of the ISO from
here as then you have the version that lets you reactivate after swapping the board as the old USB may not.
The installer can be used many times, it was the licence that comes with it that was restricted and it is now a digital entitlement. When you reinstall Win 10 you shouldn't need that key, its recorded against either your machine or your email address on MS servers now so anytime you install you just click "skip" when you reach page asking for a key, and win 10 will either reactivate on install or you need to run the activation troubleshooter to match your digital entitlement to current PC