[Eikinskjaldi]

Rather than making a mass change to the real underlying posted text you would be better adding a mod which changes the postparsed text. Once you have updated your post cache it means that every view of that text will have the formatting you want, but you are not mucking around with the original data.

If it were me I'd be adding a str_ireplace('iSketch','<h1>iSketch</h1>,$text) either directly into /includes/class_bbcode.php or as a plugin to same.

If this is still causing problems with "links on the main page" (not sure what that means) then you could use preg_match/preg_replace to perform a more stringent replacement.

However, if you really want to change the original data (and this wont help with future occurences) then you can do it via mysql

update post set pagetext= REPLACE(pagetext, 'iSketch', '<h1>iSketch</h1>');
Doing it via mysql will be
1) slow
2) irreversible
3) not solve the forward replacement problem.