I had this happen a few months ago on a SDC, although it was done online and the difference was only $9.

The website told me that I would receive a voucher for $9, but I never did. It wasn't enough to follow-up on, so I didn't bother.

The website does say that if you change fare class then "the same-day flight change fee will apply [...] plus any additional collections or refund due." however it doesn't necessarily say that you'll receive a refund - it could just be referring to the fact that the "refund" can be taken off the change fee.