XNA Game Studio 4.0 and the XNA Framework 4.0 introduced several breaking API changes. They are described at a high-level in the documentation and some of them are described in more detail on Shawn Hargreaves’ blog. These breaking changes present challenges to developers when they try to upgrade their games from XNA Game Studio 3.0 or 3.1 to 4.0. XNA Game Studio 4.0 includes a built-in upgrade wizard that will upgrade 3.0 and 3.1 projects to 4.0, but the upgrade process only updates meta-data in the project files (such as .csproj and .contentproj). Like the upgrade wizards for other Visual Studio project types, it does not attempt to make any changes to the code in the projects. This is because it can be difficult to reliably tell what the intent is of a given piece of code, and in many cases there are multiple options that could be valid solutions for breaking changes.
Fortunately, someone recently posted a cheat sheet with more details to help map compile errors that can occur after upgrading a 3.0 or 3.1 game to 4.0 to the options that you have to update your code so it will compile successfully with 4.0. If you are running into any issues fixing your game to react to breaking changes in XNA Game Studio 4.0, I encourage you to take a look at the cheat sheet at http://www.nelxon.com/blog/xna-3-1-to-xna-4-0-cheatsheet/.