If you call immediately after ticketing you may be able to get it done under the 24 hr rule. You can also explain the situation to the agent and see if they will make an exception.

I did this on another airline where a discounted coach ticket was purchased for me, but I wanted to up-fare into F (for a couple hundred extra dollars). I explained the situation to the agent (mentioning that my company will only pay for lowest cost Y, but I wanted to pay the difference). After a brief conversation with a supervisor they charged the difference to my credit card and thanked me for my business.