In some cases the platinums missed out, in other cases they cleared quite late into J.
Upgrades are generally done in the order of W->J, Y->J, Y->W. This maximises the number of pax who clear an upgrade by at least one cabin.
Having seen on various flights, who clears first and at what timeframe, they will pretty much clear everything as per the above order.
The class you're booked in also matters - eg. a pax in Y class will clear before B and H, and so forth all the way down to S. So, given similar pricing, someone booked in T class will generally have a better chance at clearing into J than someone booked in H class.