My client's outside database has changed. I have spent a few hours (fun!) matching the old and new external IDs and the internal CiviCRM contact ID as well. Not every record in the CiviCRM database has or will have an external ID.
I will probably need to do an update using SQL (mySQL/phpmySQL), but I'm a little uncertain that that's the best way and I don't want to break the entire database!
The current data is in Excel: CiviCRM ID, New ExternalID, old ExternalID.