IME, when I rebooked a lower fare, it was never sent to me, but I was able to get the code on the phone. Always ask for this after rebooking the ticket (in my case, I tried booking online, but it didn't work, so had to call in anyway, so made sure to ask what it was before hanging up). It should be available in the record.

I could never understand how they can give out an electronic voucher, but still insist that it can't be emailed, and they have to send it out USPS.