You need to check both M and J inventories if you wish to get your desired flight and get an upgrade. You won't be able book a seat on the flight with the pass if there is M0 showing. The AC.com booking screen will label the flight as sold out and the radio button will be greyed out, not allowing you to select this flight, if this is the case. If the flight is J0 and M > 0 then there is no upgrade to be had on that flight.

I haven't flown that route in a while so I can't comment on what's happening with the loads in J. I have had one flight I booked where the front was sold out and I couldn't upgrade on my pass. I checked the loads for the next flight saw that there was M and J space available on that one, changed my flight on-line and upgraded in a matter of minutes.