I had this issue a few times already, so I will write down the procedure I follow also as a reminder for myself:

  1. Replace all of the old solution name with the new one.
  2. Navigate to Properties under each project and change the Assembly Name and Default Namespace fields to new solution name.
  3. Go to solution folder and rename all project folders with the new solution name.
  4. Remove all files under bin and obj folders.
  5. Solution will not be able to load projects. Remove all projects and add them again.
  6. Re-build the project.

Source: http://stackoverflow.com/questions/21701297/error-when-renaming-asp-net-mvc-5-applciation