I currently have an Amex 241 F booking as a return and want to extend the return date but there is no BA availability. There is however availability on JAL or CX.

So what I want to do is book a normal Avios one-way on JAL or CX for the return and convert the existing booking to one-way outbound only still using the Amex 241.

Is this possible or will I have to completely cancel the original booking and remake a new one-way 241 and just hope the cancelled booking goes back into reward inventory? I really don't want to lose the outbound date.

PS. I did try searching for the answer but couldn't find this specific scenario.