In reviewing some UWP functionality I tried to open an old test project from earlier this year. It was a project to test some of the functionality announced at build this year. The functionality was made available at the time via a special SDK (version 14295) that wasn't an official/formal release.
When trying to open that project on my recently rebuilt PC, solution explorer shows that an update is required to load the project. Trying to download the update presents a dialog like this.
The solution was to manually modify the csproj file.
Above you can see that I've changed the TargetPlatformVersion to be the latest (AU) version and now the project can be loaded.
Ok, so this isn't something everyone will need but if you ever find yourself with an old project you can't open or get something someone has shared online and it won't open then this may help you out.