I am in a similar situation traveling for work and will have to book in economy JFK-HND. Was considering booking a K seat for more in one direction to then upgrade with JAL miles.
I see business availability searching on JAL with upgradeable search but wanted to confirm that I would be able to go from Economy (K) to business for 30,000 points or would I only be able to go to Premium economy for 20K.
I guess the question is are you allowed to upgrade form economy to business or only one level up.

This is the chart I see

Higher Class of Service: Premium Economy Class Required Mileage 20,000

Higher Class of Service: Business Class Required Mileage 30,000

Higher Class of Service: First Class Required Mileage 45,000