Sylvester withdrew from the case April 1, saying his administrative duties as chief judge of the district wouldn't allow him enough time. Sylvester assigned District Judge Carlos Samour to take over.
Samour must approve the new plea before Holmes is allowed to enter it, and he already has said he would require an explanation before he does so. Steinhauser said she doesn't think that will be an issue.
Samour said he would hear arguments from the defense and prosecutors about the new plea at a hearing Monday.
If the judge does accept the plea, Holmes would be sent to the state mental hospital, where doctors would determine whether he was sane at the time of the shootings. If the doctors do determine Holmes was insane, a jury could still find him guilty.
Holmes' attorneys repeatedly have said in court hearings and documents that Holmes is mentally ill. He was being seen by a psychiatrist before the attack.
Holmes had sent the psychiatrist a notebook that media reports said included crude drawings of violence. Prosecutors might renew their request to see the notebook, because state law gives them access to some medical records of defendants who plead not guilty by reason of insanity.
Prosecutors backed off their previous attempt to see the notebook when Holmes' lawyers said it was protected by doctor-patient privilege.
It's unclear how long the mental evaluation would take, but it would further delay the proceedings, which already have taken nearly 10 months. The trial is not scheduled to start until Feb. 3, and even that likely would be pushed back by the mental evaluation.
Tom Sullivan, whose son Alex was killed in the attack, said prosecutors have warned victims that an insanity plea would delay the case.
"We're just mortified that this is the process that we're going through, and we still have a long way to go," Sullivan said Tuesday. "I know justice will win out in the end."