Ever since ASP.NET MVC 3 RTM’ed, I see a lot of questions about upgrading applications using ASP.NET MVC 2 to MVC 3. I suppose many developers are unaware of a free application released by Microsoft that upgrades ASP.NET MVC 2 applications to ASP.NET MVC 3. The tool only supports Visual Studio 2010 solutions and MVC 2 projects targeting .NET 4. Please note that this tool does not support Visual Studio 2008 solutions or MVC 1 projects.
Note: ASP.NET MVC 3 can be installed side by side with ASP.NET MVC 2 on the same computer
Microsoft has also released notes that list the steps if you choose to ‘manually’ upgrade an existing ASP.NET MVC 2 application to MVC 3. Check the ASP.NET MVC 3 Upgrade Notes here