This is probably route dependent (I have been doing R->U upgrades between YYZ and HKG consistently for the past 5+ years), but I would say I score most (like 85%) of my redemption upgrades within the last 4 days of departure after the waitlist has expired. U class seats do get released within the last few days up to departure. I usually monitor ExpertFlyer loads "crazily" during the last few days, and call every time I see a change (increase) in the selling loads. Many times as soon as the loads change I called and got the upgrade seat. This goes all the way to as close as T-3 hours that I got a seat when the load went from J1C1D1I1 to J2C2D2I2. Interestingly while I was trying to ticket the redemption upgrade, I saw that I got an op-up and managed to call MPC back to cancel the redemption upgrade.