The release date of Windows 10 is less than 40 days from now.
Of course Microsoft has been providing developers with tools to help them begin the process of transitioning apps to the new Universal Windows Platform (UWP) but much like the release of Windows 10 Insider Preview program builds the process of releasing updates has been slow.
In order to help developers out Microsoft has announced plans to significantly pick up the pace for the release of the Windows 10 developer tools for Visual Studio 2015.
...starting this month, we will release previews of the Windows 10 SDK more often to our Windows Insiders. Our goal is to release a Windows 10 SDK preview that corresponds to OS flights delivered through the Windows 10 Insider Preview Fast Ring. As part of these preview releases, we will also release a matching version of the Windows 10 Mobile emulator. While we’ll do our best to align with the OS release schedule, it is possible that not every OS release will bring an updated SDK.
By matching the release of the developer tools to the latest Windows 10 Insider Preview build it means your apps can access the most recent updates on the OS side as you develop and perfect your apps for the new OS.
Of course there is the standard warning about possible bugs and that final functionality may be different than what you see in theses preview builds - both the OS and the Windows 10 developer tools.
Any existing apps you have in Windows 8.1 will run in Windows 10 so you do not lose that compatibility while you wait to transition that app to the new Windows 10 UWP.