heres what i did with my $50 expiring credit last year. Had a week booked a LAX-LAS cheapie and kept changing it until 2 days before it expired.
Flight got cancelled, called and told them I cant make it anymore and wanted to change the date into the future. Changed it to a date that I could fly a few months later. Super had to come in and make it happen since the credit was going to expire but rules are rules, if there is a cancellation they have to re-accommodate you.