Oh, wait. I think I figured it out for myself. Audra McDonald played the Beggarwoman in the "today" scenes and the other actress played her in the flashbacks. That dawned on me five minutes after I posted my puzzlement.
I should also mention that I fear it's indicative of how Republicans in Congress are gunning for PBS that it was deemed necessary to bleep the word "shit" the few times it was uttered. Not long ago on another Live From Lincoln Center broadcast, Beth Behrs from the 2 Broke Girls sitcom performed "Tits and Ass" from A Chorus Line…only she didn't. They made a joke out of her having to substitute other rhyming words for the "t" word. (Yes, I know: The song is actually called "Dance, Ten; Looks, Three." But what I called it is what everyone calls it.)
Also, before anyone writes in, I'm well aware that it wasn't a question of Stephen Sondheim buying his own tickets to the performance. I'm sure they gave him his for half-price.