I've started building my first windows 10 app

No, I don't have the SDK but that doesn't matter.

I'm building an app that will exclusively target the phone but requires capabilities that aren't available today (in 8.1) but do currently exist on the desktop for Windows apps. 

I'm assuming that what Microsoft are telling me about being able to build one app that can run on all devices and the convergence of APIs will mean that the capability will be available on the phone when the Windows 10 SDK is available.

Planning is the most important step in development anyway so doing most of the work before the API is available (but assuming it will be like the existing desktop version) means I'm not blocked by a lack of SDK.

I want to be ready for when the tools and store become available so I can make the app available as soon as possible.

What if the API is very different or doesn't allow the functionality I want when it is released? 

I'm allowing for some level of difference in the APIs so am not expecting it to be completely painless when the tools are available. 

If it's a very large difference and requires massive effort I can reevaluate then. 

If the app won't be possible when the SDK is available I'm not losing a massive amount of effort as it's a simple app. 

I think the risk is worth the initial effort and I'm learning things now in terms of having to think about what the app should be like and how I will design it for a great Windows 10 experience.