This is what I mean:
<?php
$tochange = get_posts('numberposts=-1&post_type=any&category_name=category-name-you-want-to-change');
foreach($tochange as $post){
setup_postdata($post);
$contentchange=""; /*here you do the magic, string replace or whathever you want to do*/
$tochange = array();
$tochange['ID'] = $post->ID;
$tochange['post_content'] = $contentchange;
$out = wp_update_post($tochange);
unset($tochange);
}
wp_reset_query();
?>