MySQL – how to search and relace some data ?

Hi,
Some themes use the comment author’s email to style the comment differently if the comment author is the blog admim or the post author (as apart from a passing-by commenter).
That turned out to be a problem for me when an author in the blog changes his email address, because his comments will not be styled anymore.

I would need an SQL query to search for the former email and replace it with the present email so i get the styling back. Would someone care to help, please?
(i can run SQL statements in PHPMyAdmin, and yes, i sware i will backup my database first).