While securing the same or similar seat on your new flight is ideal, the chances of that happening during irregular operations are slim. And the agent rebooking you may not be immediately aware you purchased a better seat.
First, speak up! Politely let the agent know you purchased a better seat on your original flight and they'll do their best to accommodate you.
If there's no preferred seat available on your new flight, most airlines have a process to refund the fee.
Refunds are due to passengers who purchased a better seat and don't end up flying in one, according to American, Delta and United websites. But airport agents don't process the refunds. So be sure to save all documentation — ticket receipts and boarding passes — to support your claim after you arrive at your destination.
American and United fliers who purchased a preferred seat and are impacted by a seat change to a non-preferred seat will automatically be refunded the fee to the original form of payment. On Delta, an online refund request form is available. Regardless of the airline you're flying, keep your documentation until the refund comes through.
And consider planning ahead when you purchase that extra legroom seat. Take a screen-print during the booking process showing your seat location and additional fee. It might come in handy if there are any discrepancies processing your refund.