Got $ off discount code because of flight cancellation, and the code is going to be expired, however I don't have any travel plan yet.

Should I book a ticket in summer 2018 with the discount code?
Later change the ticket to the right time/itinerary.
Will the discount still good or I will lost the discount code.

Eg. Buy a ticket with discount code $300 + $50
Pass the expire date of discount code
Change to a ticket which $370

what's the policy for this situation? $20 or $320 needed for the change?

Any idea?