Thanks for the reply, caco3...

I thought of maybe doing something similar to an export + edit + import by doing a backup, a search and replace, and then restoring the backup. Would something like that work? How dangerous is it?

The reason I don't want to use a sync to Outlook or anything else is that syncs use different "mappings", for example, what shows up in the N900 as "Mobile (home)" could show up as "Car Telephone" in Outlook... So I'd rather do as little of these as possible.