You forfeit the discount code because your new itinerary (none) does not qualify for use of the discount code. If you want to preserve the discount code, use the change option as the agent suggests. If you're Gold/75K and have no change fees, you could book a placeholder itinerary in the meantime, and change that again when you have more solidified plans. The emailed ticket numbers/value preserves it too, and would allow you to waive the change fee (I believe) if you do reach Gold/75K status at a later time. You could also try to see if another agent will reissue the code for you, but that would be a service gesture and not inline w/ policy.