If you're talking about Microsoft's vision of everything is Windows, then I'd say there's a good possibility that the Band gets a big update (OR any future Bands would have) that's called Win 10 for Band.
At this point, Windows 10 isn't really an OS. That's just part of it. Windows 10 is a family/generation of devices that all run in a somewhat unified environment. So, if the Band did get such an update, it'll be like how the IoT version runs on Raspberry Pi. It isn't desktop Windows or anything like that (not really any GUI at all in that case). It'll be a specialized thing that they call Windows 10 because it's in the Windows 10 family.
But don't expect any insane feature upgrades, the band is more or less WYSIWYG.