I think the probable answer is that they did not change the ticket. They left the ticket as originally issued, but checked you into the BA codeshare flight and lifted the original coupon. It would be as if you just showed up at the airport and said you wanted to fly on BA xxxx and here's an old ticket that's valid for this route that I want to use to pay for it - the fact that the ticket happens to be for the same flight on the same day but on another carrier's flight number is coincidental for the purpose of determining the ticket's validity.

Remember that for the ow RTW tickets, they don't need to be re-issued for change of flight, date, time, OR CARRIER, as long as the city pairs remain the same. As long as the city pairs are the same, the coupon for a particular segment is valid for any ow flight (including codeshares where both the operating and marketing carriers are ow), without needing the ticket to be reissued or endorsed.