if you can changed it during OLCI, then you should be fine. But I recall there was a thread where someone reported you can't do that anymore. I never tried myself, so I do not have any experience. You can also try to get the agent do that, but the following thread said no:
http://www.flyertalk.com/forum/air-c...ng-united.html
This very much interests me, if someone could verify that would be appreciated.