I previously posted a question regarding earning AA miles when flying BA/QF on a OW RTW ticket. Someone very cleverly noted that AA will start codesharing some of the BA flights so that 100% AA miles could be earned (say) from SIN-LHR.

Does anyone have an idea about how easy it would be to switch from a BA to an AA flight number? The flights are filling up, so I certainly don't want to wait for the codeshares to come along. I am thinking of booking on BA, and (since changes to date/time don't cost anything) switching to an AA flight number later. If this won't work, then I'd rather book on QF now instead of BA.