1.2.1.1. Before You Begin
Before you initiate this manual procedure, you should back up all components of your project, including the game code and the content pipeline. Save a copy of this data in a backup directory before you upgrade your project.
1.2.1.2. Performing the Upgrade
To upgrade your existing project to XNA Game Studio 3.0 (Preview), perform the following steps:
Open the game project file (.csproj) with a text editor such as Notepad.
Near the top of the project file, between the <PropertyGroup> XML tags, find the specification <XnaFrameworkVersion>v2.0</XnaFrameworkVersion>. Change this to read <XnaFrameworkVersion>v3.0</XnaFrameworkVersion>.
In the <Reference> statements for the XNA Framework, change all "Version" attributes from 2.0.0.0 to 3.0.0.0. For example, the following line:
<Reference Include="Microsoft.Xna.Framework, Version=2.0.0.0, Culture=neutral, PublicKeyToken=6d5c3888ef60e27d, processorArchitecture=x86">
should be changed to read:
<Reference Include="Microsoft.Xna.Framework, Version=3.0.0.0, Culture=neutral, PublicKeyToken=6d5c3888ef60e27d, processorArchitecture=x86">
In the <Import> statements for XNA Game Studio, change all path references to subdirectories for the target files that are specified in the "Project" attribute from "v2.0" to "v3.0". For example, the following line:
<Import Project="$(MSBuildExtensionsPath)\Microsoft\XNA Game Studio\v2.0\Microsoft.Xna.GameStudio.Common.target s" />
should be changed to read:
<Import Project="$(MSBuildExtensionsPath)\Microsoft\XNA Game Studio\v3.0\Microsoft.Xna.GameStudio.Common.target s" />
Save the game project file.
Open the content project file (.contentproj) with a text editor such as Notepad.
Repeat steps 2 through 4 and save the content project file.
Open the project or solution in Visual Studio 2008. The conversion wizard will begin. Allow the wizard to upgrade the project and solution from Visual Studio 2005 to Visual Studio 2008.
Copied straight from the XNA 3.0 Readme it helps!