Flanker2

My mother got the information at the airport, I believe it was HND before a HND-LHR flight.
20.000 miles is if you purchase an upgradable fare, it's not applicable to special nor standard economy fares.
As nobody purchases an expensive upgradeable fare without the intention of using it to upgrade long before the trip begins, this airport upgrade promotion is rather useless.
They need to open it up to all fare classes.

The upgrade to premium economy is not a steal at 15.000-20.000 miles considering that it's the same meal and the seats are marginally better and we're talking one-way.
Even if it were open to all fare classes, I doubt that people would be queuing up to get the upgrade. People with at least Sapphire status should actually be upgraded for free anytime there are seats available, since this is what NH does for its Gold members.

The upgrade to business class for 33.000 one-way is also not a steal, since it's not easy to rack up that many miles especially for those flying frequently in economy class fares, and the promotion for fewer miles doesn't apply on the busy routes like HND-LHR or HND-CDG. Also, JL offers award tickets for 42.500 miles "for some days of the week", which is not much more than 33.000.
Last but not least, it's subject to availability and given how busy the premium cabins are, there are not many empty seats available at all.
Given high entry mileage barriers and low availabilites, the conditions are too restrictive and they should open it up to make all fare classes eligible.