What is the test for replacing a defendant's attorney?

'"unless the defendant has shown that a failure to replace counsel would substantially impair the defendant's right to assistance of counsel."' [Citations.] Substantial impairment of the right to counsel can occur when the appointed counsel is providing inadequate representation or when 'the defendant and the attorney have become embroiled in such an irreconcilable conflict that ineffective representation is likely to result [citation].' [Citations.]" (People v. Clark (2011) 52 Cal.4th 856, 912.)