I did a roundtrip on BA London-Paris return the next day. No FF numbers given at time of reservation. When checking in for the first leg, I gave in my CP FF number. On the return leg, I gave my AA number and the agent noted that my CP number was already in the system and that she will remove it and put in my AA number. The boarding pass was printed with the AA number.

Three days later - BOTH the outbound and return flights have been credited into Canadian Plus.

How can this be? Ughhhhhh! Rats - I'd have to do another BA sector somewhere...