I recently booked a mixed award using Asia Miles which is part on CX and part on BA.

When booking, I was told that if I needed to change the BA sectors it was a free change but because it involved partner award it would invoke a reissue, so they would ask for credit card details again. When reissuing, they would refund taxes paid and then charge them again. I thought it was weird - but no choice but to accept if I wanted to redeem.

Maybe they want to do this with your ticket.