Pat,
Be careful doing this server side and make sure you have a good backup. If I understand you correctly, I would mirror the site to my harddrive, make the changes locally (while maintaining a copy of the original files) and then ftp the changed files back to your server. You can use a text editor like
EditPro , open all the documents, and do a replace all.
As DD says, Cleo probably has a better solution, but from experience server side editing, as in all editing, is not very forgiving.