Armstrong told The Associated Press early Friday in an email that Amber Neben challenged USA Cycling's decision this month to enter Armstrong as one of two American riders in the women's time trial.
Armstrong, the time trial gold medalist at the 2008 Beijing Games, says Neben won her appeal.
Armstrong, a Boise resident, learned she'd been replaced only after her plane landed in Denmark on Thursday.
Neben placed second at the national championship time trial in June, with Armstrong finishing third.
Neben was already on the U.S. world championship team for the road race.
USA Cycling director of athletics Jim Miller didn't return a phone call.