I purchased an one way Choice fare for $340 a few weeks ago. Today, I'm seeing a Choice Essential fare for $342. I called to ask if I could upfare, because I thought upfaring was possible, without a difference in fare. The PHX-based call center rep said after consulting her help desk, that it wouldn't be possible without a full reissue and penalty.

Is this true? I thought upfaring was possible by only paying the difference in fare.

Thanks!