Questions??? Answers is a little section where I pull the answers I have given to developers about actual questions they have asked. I am always open to questions, ping me on Twitter, or ask below.
I will continue to build for Windows 8.1 until the tools are fully ratified and the kinks are worked out, usually takes few months or so past release. Once I see that the numbers of phones updated to 10 are higher, I wills switch my development efforts over to strictly focus on Windows 10.
That being said, any app that I decide to develop that needs the features provided by Windows 10 SDK and is not available on Windows Phone 8.1 SDK, I will, of course develop it right away on Win10. This will also go for any apps that I am making that will have a tablet or Windows desktop focus.
As for converting apps to Windows 10, with the backwards compatibility going all the way back to 7.8, I will be converting all apps that make sense based on features and/or user numbers.
During the time between Windows 10 Phone release and general common availability, I will be definitely be working to experiment and extend my libraries to focus on the new available features to make sure I am staying current and fluent in the new SDK.