Originally Posted by jsloan

PZ (the booking class used for upgrades, including MUA) was available on the red-eye.

Currently the same-day change feature is allowing/forcing people to change into open PZ if they have a cleared upgrade. (This is also why it was the only flight offered).

Thus, no charge when you use the same-day change feature, but a fare difference (and a refund of the miles + copay for your MUA) if you use the regular change.