In the SQL Reporting Service Database you will find a table called Subscriptions, and in that table an ntext field called ExtensionSettings. This is where the messages are stored (as part of an XML document). As Sue_H mentioned, monkeying with this manually is risky. I've done it before, but I wouldn't recommend taking it lightly. Back up the database. Make a copy of the table, and copy the ExtensionSettings field into a new field of XML datatype. Try creating some update statements with SQLXML, Xpath, etc (whatever you're most comfortable with) and see if you can get it to do the update you want (safely). Once you're SURE it's correct, back up your database again, try updating the ExtensionSettings field in the live copy of the Subscriptions table, and get ready to restore your backup when it all breaks.
If it's only 100 subscriptions, I'd recommend manual copy/paste using the web front end as almost as quick and much safer.