Judge Carlos Samour decided there is "good cause" to consider allowing Holmes to change his plea to not guilty by reason of insanity and set a May 31 hearing, The Denver Post reported.
A not-guilty plea had been entered on Holmes' behalf in March when his attorneys declined to enter a plea for him. But the defense lawyers said they now have a diagnosis and opinions by qualified psychiatric professionals, leading to the change in plea request, the newspaper said.
Holmes is charged with 166 counts of murder, attempted murder and other counts in the July 20 attack at the Century Aurora 16 movie theater. Twelve people died and 58 were wounded in the shooting spree.
Insanity pleas may be entered after the arraignment if a defendant can demonstrate "good cause" for the change, Colorado law allows, although it doesn't specify what "good cause" means, the Post noted.
Colorado Supreme Court opinions indicate judges have broad discretion on whether a defendant can change a plea and that judges should err on the side of allowing the change, the Post said.
Denver defense attorney Dan Recht, who is following the case, said if Samour were to deny Holmes a chance to change his plea, an appeal likely would follow.
"It's a foregone conclusion that the court will allow Holmes to change his plea to not guilty by reason of insanity," Recht said.