I am developing for Windows 8 since before the release of Windows 8.1. My project is targeting Windows 8 devices. As Windows 8.1 is about to hit the market, do I have to change the platform of my project? Can I continue to develop it as it is? How long do I have until I must change?
The short answer is yes, there are quite a few big differences in Windows 8.1 than Windows 8.
One of the biggest ones is the lack of a LayoutAwarePage. Your app will need to accommodate almost any screen sizes and widths, which will mean that you will likely need to do all of your layout using relative spacing instead of absolute spacing (which you should be anyway). There are quite a few other changes, but many of them are tools given to the developer to make things easier or better. You should read them though, as you'll need to make the move eventually
Links thanks to @Xyroid - Migration Windows Store App based on 8.1. does it has backward compatibility with 8?, Resources for Migrating to 8.1 & Upgrading Windows 8 Apps to Windows 8.1 - Channel 9.
The links he has provided are truly top notch. Definitely check out Resources for Migrating to Windows 8 (the second link). It is from Jerry Nixon's blog, and is a great set of resources. Also, the Channel 9 videos from Build 2013 are an amazing set of resources. They will also likely have subtitles in case English is not your preferred language (at least, many Channel 9 Build videos in the past have).