If you're on unix and have the standard find and rename utilities installed, this should work for you.

% cd YourDocRoot
% find . -name '*.html' -exec rename .html .shtml '{}' \;
[download]

Though you should test it in a temporary directory first.

Update: Here is a faster version using xargs

% find . -name '*.html' -print0 | xargs -0 rename .html .shtml
[download]

-Blake