You should be able to. Pull a search with customer internal ID, address internal ID, Default Shipping Flag and Zip. And any other fields you need to ID the addresses you want to update. Then do an update CSV import mapping customer internal ID, address internal ID and Zip. That should do it. Mapping the Address Int ID is very important, otherwise the import tool treats what you're doing as a new address record. Learned that lesson the hard way in my early days of NetSuite....