dbuckho - I want to be sure you're not confusing EQMs (elite qualifying miles) with EQPs (elite qualifying points). You state that you need around 2500 additional American miles. This makes it sound like you are trying to qualify for AA elite status using miles and not points. Even though either 50,000 miles or points will get you PLT - there is a difference between the two. You cannot spend points and you don't get any kind of PLT bonus on points. Unless you've done a bunch of paid premium class travel this year (in which case your YTD point balance will be considerably higher than your YTD mile balance) - you shouldn't be worried about how many points you'll be getting on your CX flights. With this in mind, H fares on CX (technically a discount economy fare and cheaper than Y or B) will also net you 100% AA EQMs (but only 1.0x EQPs). Thus, H fare should do the trick for you and save you a little bit of money.

Also, for what it's worth - I know that AA lets you upgrade the fare class of a single segment like you want to do (although when I was a PLT they charged me an extra fee so I could have the priviledge of paying them more money for the same seat!). So, from a technical standpoint, I think it would be possible to do on CX.

-FlyerBeek