The defendant was convicted of armed robbery while masked. The Appeals Court reversed the conviction, Commonwealth v. Knight, 16 Mass. App. Ct. 622, 623 (1983), but this court affirmed the conviction on further appellate review. Commonwealth v. Knight, 392 Mass. 192 (1984). The defendant subsequently filed pro se in the Superior Court a motion for new trial and a motion for appointment of counsel, which the trial judge denied. The defendant appealed from the trial judge’s denial of his motions, and we granted the defendant’s application for direct appellate review. We affirm.

*623At trial, the defendant moved to prevent the Commonwealth from impeaching the defendant with prior convictions should the defendant testify. The trial judge declined to exclude the prior convictions. The defendant, who did not testify at trial, appealed. This court considered “[t]he narrow question . . . whether the judge knew he had discretion and exercised it, or believed that his decision was compelled by [G. L. c. 233, § 21].” Commonwealth v. Knight, supra at 195. We concluded that the judge was aware that he had discretion to exclude the prior convictions, and had exercised it. Id. The defendant did not argue on direct appeal that this court should substantively review the judge’s exercise of discretion, instead confining his argument to whether the judge knew he had discretion and had exercised it.

Subsequent to the defendant’s trial and our decision affirming his conviction, we decided, in Commonwealth v. Maguire, 392 Mass. 466 (1984), that the trial judge’s exercise of discretion may be reviewed by appellate courts. Id. at 470. The defendant argues that our decision in Maguire, decided after his trial and appeal from his conviction, should apply retroactively, and requires that his motion for a new trial be granted.1 We disagree.

A judge of the Superior Court may grant a new trial “if it appears that justice may not have been done.” Mass. R. Crim. P. 30 (b), 378 Mass. 900 (1979). “In deciding whether to grant a motion for a new trial, the question whether ‘justice may not have been done’ at trial is left largely to the discretion of *624the judge who presided over the case.” Commonwealth v. Pope, 392 Mass. 493, 497 (1984).

Maguire affected the scope of appellate review, not the standards for the trial judge’s exercise of discretion whether to permit impeachment with prior convictions. Maguire did not alter the standards that trial judges apply, which had been stated prior to the defendant’s trial. See Commonwealth v. Diaz, 383 Mass. 73, 80 (1981) (“In a criminal case, the court shall have discretion to exclude evidence of a prior conviction offered to impeach the credibility of the accused if it finds that its probative value is outweighed by the danger of unfair prejudice”). See also Commonwealth v. Chase, 372 Mass. 736, 750 (1977).

In Commonwealth v. West, 357 Mass. 245, 249 (1970), overruled in Commonwealth v. Maguire, 392 Mass. 466, 470 (1984), this court interpreted G. L. c. 233, § 21, as then amended, to give the trial judge no discretion to exclude evidence of a witness’s prior convictions. In Commonwealth v. Chase, however, we noted the prejudice which could result from impeachment of a defendant with prior convictions similar to the crime charged, and stated, “[W]e would not deny the right of a judge to avoid any question of unfairness by excluding such evidence in a situation where the likely prejudice to the defendant is most intense.” Commonwealth v. Chase, 372 Mass. 736, 750 (1977). Subsequently, the “right” mentioned in Chase became a duty of the trial judge to exercise discretion in admitting prior convictions, but it was not until our decision in Maguire that we “granted a defendant appellate review of a judge’s discretionary ruling admitting evidence of a prior conviction, where ‘the judge exercises discretion and there is no unfairness in a due process sense.’” Commonwealth v. Maguire, supra at 470, quoting Commonwealth v. Knight, supra at 194. See also Commonwealth v. Diaz, 383 Mass. 73, 80 (1981).

In Maguire, we reasoned that appellate review is available because “[a] rule that denies any appellate review of the exercise of discretion ‘would discourage uniformity of treatment of defendants, and should be avoided. ’ ” Maguire, supra at 470, *625quoting Commonwealth v. Martin, 392 Mass. 161, 163 (1984). The rule announced in Maguire was not addressed to remedy infirmities in the conduct of criminal trials, but instead to further the orderly administration of justice. Even if we consider Maguire as extending some level of additional protection to defendants, we note that Maguire represents an incremental change in the protection afforded a defendant rather than a wholesale revision of prior practice. The standards that trial judges are to apply in exercising discretion were well established prior to Maguire. See Commonwealth v. Diaz, supra at 80. Although a trial judge’s exercise of discretion might be affected marginally by the knowledge that it will be subject to appellate review, we are unwilling to assume that trial judges misapplied the standards set forth prior to (and unaltered by) Maguire solely because they thought their exercise of discretion was free from appellate scrutiny.

We conclude that the rule announced in Maguire did not substantively affect the application at trial of the standards for admitting prior convictions, and the defendant is not entitled to relief based on the rule announced in Maguire. Accordingly, the trial judge did not err in denying the defendant’s motion for a new trial.

Orders affirmed.