MacBook Pro with Retina Display Teardown - Page 3 - iFixit:
MacBook Pro with Retina Display 15" Mid 2012 Repairability Score: 1 out of 10 (10 is easiest to repair).
—Proprietary pentalobe screws prevent you from gaining access to anything inside.
—As in the MacBook Air, the RAM is soldered to the logic board. Max out at 16GB now, or forever hold your peace—you can't upgrade.
—The proprietary SSD isn't upgradeable either (yet), as it is similar but not identical to the one in the Air. It is a separate daughtercard, and we’re hopeful we can offer an upgrade in the near future.
—The lithium-polymer battery is glued rather than screwed into the case, which increases the chances that it'll break during disassembly. The battery also covers the trackpad cable, which tremendously increases the chance that the user will shear the cable in the battery removal process.
—The display assembly is completely fused, and there’s no glass protecting it. If anything ever fails inside the display, you will need to replace the entire extremely expensive assembly.