\begin{equation*} A = xyzp \end{equation*}
\begin{equation*} A = x\overbrace{yz}^{B}p \end{equation*}
This should have been an \onslide* question according to the subject. And yes, you can use that to do the thing. \begin{equation*} \onslide*{1}{A=xyzp} \onslide*{2}{A = x\overbrace{yz}^{B}p} \end{equation*}
It's as simple as that. Don't try to make more efficient things with \bgroup etc. That'll just cost you and us time.
-Hendri.