One of the Court TV panelists explained that in New York the judge would have to get approval from the defense in order to replace a juror. If no approval it's a mistrial. Laws are different for California.
I'm betting the juror replacement has to be approved by the defense, i.e., no juror can be placed unless the defense has approved him or her. It's the process of selection that occurs before the jurors and replacements are told they will be accepted. Sometimes it's a tricky little process and every state has it's own little quirks.
It's NOT the Defendant, the record has to have EITHER the objection or the consent of the Defense Attorney on the record. A defendant charged with double murder is consulted about what he WANTS? I don't think so.