Without knowing the specific fare bases for each segment, can't answer the question for you, but the return portion of a discounted Y ticket with the outbound in J likely isn't worth that much. The change fee may well be in the $300-450 range.
If you're only going to see $200-300 from the ticket and there's a $400 change fee, forget the whole thing. Just no show.
As others note, if the flight is cancelled or there is a significant schedule change or delay, you may cancel for a full refund with no penalty and to your original form of payment.