I am usually more of a OW flyer, but because of the attractive price, I purchased a MAD-YUL ticket in business (Z fare) with KLM. The fare conditions also mentioned no change fee, only fare difference. It happens that I need to change this ticket. I went to the KLM website and tried to change the dates after verifying there was Z-inventory. If I try to change only the return date, it seems to work well. If I both change the departure and return date, I can get past selecting the new departure date but I then get an error: "No flights available for this combination...". I have tried literally dozens of combinations without success. I have called KLM and after spending close to two hours with an agent, the best he could offer me was a 320$ fare change, when the sum of changes on the website (when taken separately) was 12$! The most frustrating part is that the fare combination seems widely available for purchase at the same price I paid and same booking classes for the dates I am interested in.

Am I missing something obvious? Should I start by changing the return date and then try to change the departure? Should I accept the offer I got over the phone? Please advise...