does anyone know exactly when cx does its operational upgrades?

i have been very lucky with op ups from cx (i'm running at about 30%, biz to first). my situation is a bit odd as i am current exp with aa but am about to switch to qf's program and will be a qf and oneworld "nothing" for a few months until i accrue qf platinum.

i have a biz class syd-hkg at the end of the month enroute to mru to pick up a rtw ticket there and don't want to destroy my chances for an op upgrade by using my qf ff # but want to count the miles to my lowly qf account.

my idea (perhaps naive as i have never done this before) is that i would check in with my aa ff # (exp) and then change it to my (lowly) qf number at the gate right before i board.

do you think this would this work? if so, how should i time the ff # switch-a-roo?