Is a defendant deprived of his constitutional right to effective assistance of counsel when a trial court denies his motion to substitute one appointed counsel for another?

"[A] defendant is deprived of his constitutional right to the effective assistance of counsel when a trial court denies his motion to substitute one appointed counsel for another without giving him an opportunity to state the reasons for his request. A defendant must make a sufficient showing that denial of substitution would substantially impair his constitutional right to the assistance of counsel [citation], whether because of his attorney's incompetence or lack of diligence [citations], or because of an irreconcilable conflict [citations]. We require such proof because a defendant's right to appointed counsel does not include the right to demand appointment of more than one counsel, and because the matter is generally within the discretion of the trial court." (People v. Ortiz (1990) 51 Cal.3d 975, 980, fn. 1.)