I have had some more instances of scrambled screen lately, but not entire screen (had a few in the past) I will say two things though.
One, the switching between IGPU and DGPU seems to take longer, the popup bubble appears, hangs there, the I changes to D, etc., and back again.
And now that I have upgraded to yosemite, I get these total freeze, BLACK screen hard reboots which I never really had before. and they are happening every other day.
Does this seem to be GPU related? I have the mid-late 2012 machine rMBP and could be covered under warranty.. but don't want to lose my sammy screen (originally swapped out machines that were poor LG)
Impossible to tell if the crashes are GPU related.
However, don't worry about the screen chances are new screens are going to be a lot better than the original 2012 screens (which had all sorts of issues, they were the first retina laptop screens on the market). These are no longer the same screens.
I have no complaints with my new screen though honestly I didn't check it in detail either. A friend of mine had his screen replaced 7 times on the original 2012 rMBP... both Samsung and LG had their issues back then. New screens are better.