To further add to the confusion, it is also not true that all Y fares are upgradeable for only 15k miles. I found that out today when I tried to upgrade DTW-TPE. According to the agent, not all Y fares are equal, and in my case, it would require 30k miles for the upgrade. Does this mean that they don't sell coach tickets on this route that can be upgraded for 15k miles?
-Jeff