I have found that if you "modify" your booking, no matter the fare category, that the discount will apply to the modified booking (provided that the modified booking complies with the original T&C of the discount, even if the "book by" date of the discount code has passed).

If, by contrast, you cancel your flight your discount code goes away too... On a Lat fare cancelling your ticket will get you a refund and ac.com will show that your discount code has already been used. AFAIK discount codes can't be used via agents if you were to elect to take a credit instead of a refund.