He is said to have been helping the country's president at the time - Ange-Felix Patasse - resist a coup bid.
"The appeal chamber has decided that the decision of the first court should be reversed," Judge Akua Kuenyehia said in announcing his decision on Wednesday.
"The potential length of sentence if he is convicted is a further incentive for him to abscond," he added.
If convicted Bemba is likely to face a lengthy prison sentence, though the court's statutes do not mention a maximum term.
Bemba, who was also a business tycoon, left DR Congo in 2007 after losing presidential elections held during a political transition in the wake of a 1998-2003 civil war.
He was arrested on an ICC warrant in May 2008, and is currently the most senior political figure in the custody of the court.
Bemba has not entered a plea ahead of his trial, which is scheduled to start April 27 next year.