Could the screen of a MacBook Pro 15" Late 2011 be upgraded?

Sorry no, the GPU is soldered directly onto the main logic board so you can't swap it out. In addition the number of lines between the GPU & the display are much more than what your current MBP has so you still couldn't swap the displays out either.

Bottom line here is you need to get a Retina system to get the higher Retina graphics. You may want to wait a bit as Apple is slated to make some product announcements next mouth.

Maybe they will intro a standard MBP with a Retina display as a lot of pros would like the display but also want expandability like having more memory than what the current Retina models offer (they have soldered in memory so you can't up it).