What is the test for reducing a defendant's first degree murder conviction to second degree?

According to defendants, this instruction essentially told the jury, contrary to California law, that juror unanimity was not required when deciding whether the murder they committed was of the first or second degree. (See People v. Diedrich (1982) 31 Cal.3d 263, 280 [trial court erred by instructing the jury that the defendant could be convicted without the jurors' unanimous agreement on a single, specific act of bribery as the basis for the conviction].) Thus, defendants contend that, because their first degree murder convictions could have been based on findings by some jurors that premeditated murder or felony murder was proved, and by others that only murder with malice aforethought was proved, their convictions must be reversed or, at minimum, reduced to second degree murder.

Relevant to defendants' challenge, when "the jury is 'misinstructed on an element of the offense . . . reversal . . . is required unless we are able to conclude that the error was harmless beyond a reasonable doubt.' [Citations.]" (People v. Wilkins (2013) 56 Cal.4th 333, 348.) However, "[t]he failure to give a unanimity instruction may be harmless error if we can conclude beyond a reasonable doubt that all jurors must have unanimously agreed on the act(s) constituting the offense." (People v. Norman (2007) 157 Cal.App.4th 460, 466.)