I came across a scenario where I wanted to return a custom collection of a model type that was more efficient to build with a stored procedure. It worked great for displaying but not for subsequent editing of the items since the return type collection didn't match the model. To fix this I found you could change the return type of a stored procedure following these steps:
  1. Open edmx model file
  2. Go to model browser view
  3. Expand the Function Imports node
  4. Double click relevant stored procedure
  5. Change return type select to Entities and pick desired model