A round-trip coach award between the US and Asia requires 70K Skymiles on SQ and 60K on NW. I'd say you're at least due a refund of 5K miles for the "downgrade" to NW on the return.

As to your original question, call up the reservation online and look at your ticket receipt (click on the E-Ticket Number on the reservation summary). What is the fare basis code for your new flights?