For now it seems to be possible to upgrade either Y to J or Y+ to J, with the same mileage requirement for each. However, I think others have noticed that once Y+ is sold on more flights that will change and it will be one-class only, so Y to Y+ or Y+ to J, but Y to J would only be available if there is no Y+ cabin on the flight.

OP, if you're trying to do a day-of-flight "instant upgrade" I think one-way may be your only option so you'd be using more miles. (Seem to recall something from AsiaMiles website about how these can't be done roundtrip.) Then again, round-trip upgrades seem not to exist anymore anyway since you have to issue the upgrade ticket for each leg as it clears, at the one-way mileage. The main thing you lose with the instant upgrade is the ability to waitlist for a few weeks before the flight, but since no one can get through to the MPO call center anyway the airport option is better than nothing.