If you have any .Net Core Preview 2 applications that use project.json, you will encounter errors if you try to update to .Net Core Preview 3. This is due to the fact that .Net Core is moving away from project.json and will use the normal .csproj files going forward. For more information on that change, see this blog post by the .Net Core engineering team.
Fortunately, the new dotnet command line utility supports migrating from project.json to the new .csproj format. However, that is not very apparent from viewing the command line help output, which normally tells you the commands available to run.
But rest assured, the command does exist, and Microsoft even documented it. I tested it out a simple console application that I had generated with Preview 2, and it worked just as expected. It also backs up your old project.json file in case you need it.