Some fans I know thought Hall was doing a Sheppard impression...but this article from the Times explains differently.
Mr. Hall, who has been the Yankees’ backup announcer since the mid-1960s, channels Mr. Sheppard’s elegant, careful enunciation when he is behind the microphone. “When I started,” he said on Wednesday, sounding less formal on the telephone, “Bob’s one requirement was that he didn’t want the fans to hear anything markedly different from what they were used to. People think I try to imitate Bob. There’s no way I could imitate that voice, but I do use that same style.”
That confuses people. “They think, ‘He’s trying to mimic’ — some people in the press or on television will use that word,” Mr. Hall said. “There’s no intention on my part to do that.” Certainly not: He and Mr. Sheppard both taught public speaking, “and we believe that you should pronounce each word carefully and not rush it the way many radio people do.”
I don't know how old Mr. Hall is, but the same article mentions a 45 year tenure at St. John's. That puts him at least 65 years old and probably older.
My guess is that the Yankees want a new long-term voice for the new stadium....not one who will do last a few years and then be old or retired. I think the odds of two straight PA announcers in their 90's are unlikely. Tonight we'll find out if Paul Olden is the man for the job.