Not sure I understand you - my understanding is that you CANNOT upgrade individual sectors on xONEx bookings using $/£/¥/€ etc, EXCEPT to upgrade to premium economy on a LONEx. The ONLY way to upgrade to the J cabin is by using points/miles from the operating airline (namely, QFF on a QF flight, BAEC on a BA flight, and so on).
Can you please clarify exactly what it is you are asking.
Dave