Exchange ticket with $ off discount code.
 

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?

If the discount code is valid at the time of exchange, I think it will still apply. I tried once to price match a ticket with discount code and the code still applied.
if the discount code is expired at the time, I don't know. It's totally up to AS's decision and the website developer. Please post your outcome if you have chance to try it. Thanks.

In my experience you'll be fine making one change. Anything after that, the website will make you call in and then the agent may or may not let you change using the discount code.