Change rel="lightbox" to class="lightbox"

Well I would need a function so that all rel=”lightbox” get changed to class=”lightbox”, for pages and posts.
I don’t want to replace them by a DB Query ’cause we’re using multiple themes and one of them is not coded in HTML5 but in XHTML 1.1 which doesn’t lists rel=”lightbox” as an error.

And also sometimes it’s rel=”lightbox[%number%]” or rel=”lightbox[gal]”.