HKFlyer -
That's why you were chosen to be upgraded but not your fiancee. You were on a full economy class ticket, and on a reservation on its own. Even though the both of you were travelling, your fiancee's reservation was seperate from yours since yours was on a full fare ticket and hers not. Your travel agent could have indicated through their reservations systems that the both of you were travelling together. It's a procedure called "TCP2" on airline computer systems .. though not many agents could bother with this!

It's also because your ticket was as close to being a Business class ticket anyway. So you would be more apt to being chosen for an upgrade.

Next time - rule of thumb, get you and your fiancee booked on the same reservation - ie same locator code. If you are travelling on seperate fares, then ask your travel agent to link the two reseravtions together. CX was right in that sense as to why they couldn't upgrade you and your fiancee together. Unless there was some 'computerised' proof that the two of you are together, they must answer to the honchos at Management why they upgraded someone from economy on a discounted ticket if the flight was not oversold.