I am puzzled by upgrade possibilities for a general member (not me) flying on UA839 LAX-SYD on 6 Jun booked in R class. Currently PZ0 with 19 as yet unallocated seats in Polaris class. She would try to upgrade with miles plus copay, but would need to buy miles. What are the chances of such an upgrade coming through? When would be the best time to buy the miles if there is a reasonable chance?