Trying to reschedule a DONE4 flight but do not see D fare class for my desired dates. I believe I can downgrade to B (economy) for this flight. Is it also possible to downgrade to Premium Economy,JAL W fare class?

After I posted this I noticed the available fare classes listed in the user guide and it seems that JAL Prem Economy is E. Anyone have any luck in getting something other than the official fare class?