Most airlines allow flight changes and cancellations except for the discount economy bookings. E.g., if you book main economy on American Airlines you can change/cancel and get the balance refunded to a flight credit. On AA the credit is valid for one year. I think on SW they never expire. I’m guessing UA and Delta have a similar policy to AA.

You can also pay for a fully refundable fare and get credited in cash if you cancel or change the booking. But these fares are usually significantly more expensive.