After reviewing the iFixit teardown of the "New" rMBP I was very surprised to see there is absolutely NO ability to upgrade anything.
Long before you were born I watched my father fix a TV he built himself (Heathkit for those old enough) by checking the tubes. As an adult, I remember dragging my set to the TV repair shop because buying a new one was just too expensive. High quality, low cost, less flexibility. So it goes with the future.
All the business pundits say Apple is too secretive. But the MBPR is a clear roadmap of where Apple thinks computing is going. It is too expensive for many now, just like the Air was when it was released. That is why there is a non-retina version. It will get cheaper. Eventually everyone will copy it. Progress, which in the computer industry means what Apple does, marches on.
But, for that old time flavor, build a Hackintosh. It's not like the days when real men could solder parts onto a breadboard, but it will do as an example of why most people don't want to mess with this stuff.