To clarify the rule because it might catch someone, unlimited changes with no OWE fee are permitted for segments AFTER the first international departure. Changing the first international departure in any way (date, flight number, airline used) requires repricing of the ticket (rather important if the fare has changed). From CAI the first segment is also the first international departure so that makes it simpler. BA charges a ticket handling fee separate from the OWE fee, something like 25 GBP, so it won't be free in any case. An option is to wait and have the ticket reissued later, in case there are other changes you want to do (the 75 USD OWE fee is only 40 GBP or not much higher than what BA would charge for the "free" reissue).