Appreciate what you say above toyotaboy, however I don't normally see any difference for me in this scenario. Interestingly, on your BP it actually will now say "CXxxx", and then in smaller print it will say something like "ticketed as KAxxxx", or vice versa (four letter codes are when you're flying on metal different from where you booked).

I frequently have this happen with PEK, for example:

*I frequently book a KA round-trip on KA metal but change my schedule so I fly on the CX flight to/from PEK
*More rarely I'll book the round-trip on CX because of the timing but then switch one or both of the flights to KA metal.

In both of the scenarios above I have definitely been op-upped. Another example, from HGH:

*If I travel HGH personally, I buy at cathaypacific.com to get the double miles for Amex (I don't think dragonair.com can do that?). Hence, I buy the CX codeshare with KA metal, since only KA flies to HGH. I have hit numerous op-ups on this route, even though I booked the CX flight code.