I was searching for one way fares (i.e., MEL-LAX) that were upgradable with the MILES-YB option. When the selection based on schedule appeared, the choices were all fares that were refundable or changeable without the penalty fees, etc. But only some were listed as B fares. The others were Q and above. I forget some of the details, but the fares only had about US$200 to $400 between them.

fjgolder