As per PCGS, 41% of crossgrades have recently succeeded. See the link below.
After some very disappointing early lessons, I am getting a better sense of what is required to achieve their grades in copper coins. In my first cross grade submission my success rate was around 25%. Just recently I sent 4 coins in for crossgrading in some very tough MS grades and all 4 made it...very pleased.
https://www.PCGS.com/Statistics/