[kh99]

It sounds to me like you might want to use SQL to permanently change the post text in the database rather than to use a plugin to change it only when it's displayed.

Maybe something like:

UPDATE post SET pagetext = REPLACE(pagetext, 'www.oldurl.com', 'www.newurl.com') WHERE pagetext LIKE '%www.oldurl.com%'

But doing things like this is a little dangerous. You'd want to back up your database first, or maybe even get someone else to do it if you don't feel comfotable with it.