In general - yes you can pay to change your booking class, I've done it a few times. As far as it not being "upgradeable", that only refers to the use of flying club miles. Cash upgrades have no restrictions.

However, given you've booked with Expedia that does change the story. Upgrading with miles is not possible at all I believe. Cash upgrades and fare class changes should be possible but I you'd need to go through Expedia to arrange that.