FWIW, I just changed all remaining flights on a BA issued xONEx ticket. The initial phone agent said they were not trained to handle this issue so quickly transferred me. I only changed date / time / flights (not cities or stopover / transfers). Read out what I wanted, the agent phoned back (from India) within about 30 minutes to confirm and clarify a few issues. All seemed well handled. Still awaiting the confirmation email to double check. So far, quite happy.