This is the screenshot from the just-opened Apple Store in Chongqing, China. You can tell from the picture that there will be 3 models of rMBP 15'. Yes, you guessed right. The upgrade is minor, 200Hz bump, and 8GB RAM added to the entry-level one. The major difference would be a "high-end" rMBP, but still, no rMBP 17'.