Attorney Daniel King made the request in court, saying the defense now has a diagnosis for James Holmes, though he didn't specify what it was.
The judge said he would consider arguments first about constitutional questions the defense has raised about Colorado's insanity law. He's expected to decide whether to accept a new plea sometime before a May 31 hearing.
Holmes had bushy hair and beard at Monday's hearing. He didn't speak at all and entered the courtroom with his eyes downcast.
The not guilty by reason of insanity plea is widely seen as Holmes' best hope, and perhaps his only hope, of avoiding the death penalty. But his lawyers have held off until now, fearing a wrinkle in the law could cripple their ability to raise his mental health as a mitigating factor during the sentencing phase.
Two judges have refused to rule on the constitutionality of the law, saying the attorneys' objections were hypothetical because Holmes had not pleaded insanity. The defense had little choice but to have Holmes enter the plea and then challenge the law.
Holmes' lawyers announced last week that Holmes would ask to change his plea at Monday's hearing.
Prosecutors are seeking the death penalty. They say Holmes, a former neuroscience graduate student, spent months acquiring weapons and ammunition, scouting a theater in the Denver suburb of Aurora and booby-trapping his apartment.
Then on July 20, dressed in a police-style helmet and body armor, he opened fire during a packed midnight showing of "The Dark Knight Rises," prosecutors say. Twelve people died and 70 were injured.
No motive has emerged in nearly 10 months of hearings, but Holmes' attorneys have repeatedly said their client is mentally ill. He was being treated by a psychiatrist before the attack.
The insanity plea carries risks for both sides. Holmes will have to submit to a mental evaluation by state-employed doctors, and prosecutors could use the findings against him.
"It's literally a life-and-death situation with the government seeking to execute him and the government, the same government, evaluating him with regard to whether he was sane or insane at the time he was in that movie theater," said attorney Dan Recht, a past president of the Colorado Criminal Defense Bar.
Among the risks for prosecutors: They must convince jurors beyond a reasonable doubt that Holmes was sane. If they don't, state law requires the jury to find him not guilty by reason of insanity.
"That's a significant burden on the prosecution," Recht said.
If acquitted, Holmes would be committed to the state mental hospital indefinitely.
A judge entered a standard not guilty plea on Holmes' behalf in March, and he needs court permission to change it. Recht said it's a foregone conclusion the judge will accept the new plea to preclude appeals later.
The mental evaluation could take weeks or months. Evaluators will interview Holmes, his friends and family, and if Holmes permits it, they'll also speak with mental health professionals who treated him in the past, said Dr. Howard Zonana, a professor of psychiatry and adjunct professor of law at Yale University.
Evaluators may give Holmes standardized personality tests and compare his results to those of people with documented mental illness. They will also look for any physical brain problems.
Zonana estimates he has conducted around 200 mental evaluations of criminal defendants, including some death penalty cases.
"All cases are tough," he said.
Meticulous planning, as in the scenario prosecutors laid out against Holmes, doesn't necessarily mean a defendant is sane, Zonana said.
Zonana said he helped evaluate Stephen Morgan, who was found not guilty by reason of insanity in the 2009 shooting death of Johanna Justin-Jinich in Middletown, Conn., where she was attending college.
Evidence showed Morgan planned the shooting, Zonana said, "but he was delusional as hell."