I have a flight from Jan 10 - Feb 3 2017 for Japan Airline

BOS - TOKYO - SINGAPORE
SINGAPORE - TOKYO - BOS

I called their contact center to see if I can change the return flight date to 3/2/17 and the rep said there were no N fare available and she needs to contact the fare agent to get a quote.

She said minimum for change fee is $250 and potentially the difference in air fare. If I have too pay too much since she said I can't just cancel the return flight, I was thinking of staying in Tokyo, skipping the flight to Boston and booking a one way back to Boston separately.

Has anyone done something like this or similar? I was just afraid they would charge me the $300 fee for no show. Or charge some fee for not being on the last leg of the flight.

Any advice would be appreciated!

Thanks.