So, a while back I made a redemption booking LHR-MAD-TFS - at the time, there was no J availability on the MAD-TFS leg, so that leg was booked in Y. Now that availability has opened up, can I alter my booking via BAEC to pay the difference in miles/fees?

MMB doesn't permit it, but I think this might be down to it being a partner booking.

Just wanted the opinion of the greater FT hive-mind, before I potentially waste a bunch of time on hold to the boys and girls in Bremen tomorrow...