The method depends upon whether and how you have used DRI...

If, as your "schema" pseudocode implies, you have not used DRI, you can just use UPDATE statements on each table.  If you don't have exclusive access to the database, then update both tables within an explicit transaction.

If you use DRI with the ON UPDATE CASCADE option, this sort of thing is easy.  Read about this in the "CREATE TABLE" topic in BOL.