1. defaults don't deal with existing records. They deal with NEW records.
2. Calculations, but for rare occasions, should not be done or stored in tables. If you are using a web database from Access 2010 or storing transactional data like tax rates and such that have to be kept as they can change over time, then those are a couple of the exceptions.
3. You would use an Update query to update the field appropriately. But I still question the keeping of calculated data in the table, especially if the fields that make up that calculation are in the same place.