Can a non-retina MacBook's screen be replaced with a Retina screen?
Not easily. The Retina MacBook Pro introduced a new display assembly which makes retro-fitting very difficult.


  Rather than sandwich an LCD panel between a back case and a piece of glass in front, Apple used the aluminium case itself as the frame for the LCD panel and used the LCD as the front glass


Source: http://www.ifixit.com/Teardown/MacBook+Pro+Retina+Display+Teardown/9493

The cost of doing so would probably be more than the rMBP itself.