When Windows 8 launched back in 2012, it came with an unsurprising bump in the NT Kernel version from 6.1 to 6.2. This is a common thing with every new version of Windows, whether the version increase is small or large depends entirely on how the Windows team are feeling at the time. When Windows 8.1 launched last year, it yet again bumped the NT Kernel version from 6.2 to 6.3, so it makes sense for the next Windows update to bump the NT Kernel again right? Well, no.
Even though Microsoft is touting Windows 8.1 as an 'update' for Windows 8, in reality it's a whole new version of Windows, hence the NT Kernel jump. The Windows 8.1 Spring update, also known as Update 1 is going to be more like your traditional update, meaning it's not an entire new OS. According to Win8China, Windows 8.1 Update 1 will not increase the NT Kernel version, meaning it'll sit at NT 6.3, much like Windows 8.1. Of course, this is only a rumor, so take it with salt.
It's currently unknown how Microsoft are planning to distribute and advertise the upcoming update. It's entirely possible that Microsoft will push the update via Windows Update and not even talk about it on a consumer level, much like your regular Windows updates. Since we don't know what's in Update 1, it's currently hard to say.
Recent rumors are claiming that Windows 8.1 Update 1 is being internally tested by Microsoft Employees already. We hear that Microsoft are planning to launch the update in April, during it's BUILD developer conference.