You're right...AA requires economy class tickets to be FULL FARE which means Y, Y2, YOX fare basis before upgrading can be done. Now who on earth buys full fare coach tickets? On the other hand, HK Flyer, if you have any remaining Passages points left over, you can upgrade from any fare (even cheapie consolidatore tickets).

For people who want to upgrade from cheap economy class tickets to Biz Class on Cathay, the programmes which allow this are AsiaMiles and Swissair Qualiflyer.