JL as a matter of principle only give opups when Y is over booked. They are happy to fly with an empty J and full Y. So you should look at Y loads to assess the likelihood.

There are no discounted cash upgrades to J, but to PY you can pay 10.000 to 30.000 yen depending on route if seats are available. (e.g. SIN is 10K LHR is 30K)

JMB members can use points to upgrade in advance or at check-in, it does require a booking class from K and upwards though.