How to convert \pmatrix to \begin{pmatrix}

I would like to convert \pmatrix to \begin{pmatrix}.

I found a solution for \case here where it is said that this can be done by

\let\amscases\cases\makeatletter\def\cases{\@ifnextchar\bgroup\plaincases\amscases}\def\plaincases#1{\begin{cases*}#1\end{cases*}}\makeatother

But I haven't the least idea how to modify this for \pmatrix?

\documentclass[12pt]{article}\usepackage{amsmath}\begin{document}\begin{equation}\pmatrix{a & b \cr         c & d}\end{equation}\end{document} 

Gives the error massage:

Package amsmath Error: Old form `\pmatrix' should be \begin{pmatrix}. \pmatrix

I'm seeking for a solution like

\let\amspmatrix\pmatrix\makeatletter\def\pmatrix{\@ifnextchar\bgroup\plainmatrix\amspmatrix}\def\plainmatrix#1{\begin{pmatrix*}#1\end{pmatrix*}}\makeatother

since I have a document with hundreds of the old plain TEX pmatrices which would be a horror to replace them by \begin{pmatrix} \end{pmatrix}.