I've said this numerous times regarding college football replays, but it is every bit as valid in baseball.
You don't need a current or former umpire working the replay booth. Umps/Refs have a natural predisposition towards protecting their own, even more so than cops. The calls that are reviewable are not plays that require an umpire's judgment - anyone with experience in the sport in question can review to see if the play was called correctly. Penalties in football, and balls and strikes in baseball, are not reviewable plays anyway. It doesn't take a skilled umpire to watch a play on video to determine if a tag was made, if a runner beat the ball on a force out, or if a potential home run stayed fair or cleared the yellow line.
I'd prefer to see retired players, former coaches or managers, or executives making the final judgment on reviewed plays. I guarantee the rate of overturned plays would go up, as would the accuracy of the decisions. Allowing officials to review their own mistakes is like letting convicts be judges for criminal trials.