CCK: Propogating new field into old nodes

Adding a new field (esp. via Computed Field) to a CCK node does NOT add that field to EXISTING nodes, or, if it does, it makes the value NULL. (A node having a NULL field value causes any table to not show that node's row, among other bad side effects.)

Is there any way within Drupal's GUI to back-propogate a new computed field's "result" value, or at least its default value, into ALL existing nodes (potentially thousands) of the corresponding CCK type?

If there's no way to do this within Drupal's GUI, can somebody who knows SQL tell me the SQL query I need? (I'll use phpMyAdmin to execute it.)

Thank you!