Convert Existing Body Fields To CCK Text Fields
Three options spring to mind:


Views Bulk Operations. Safest if the amount of nodes to convert is manageable. You'll probably need to use custom PHP to make the conversion happen though, as the ready-made functionality won't cut your demands.
Custom script where you load each node to be converted, modify the structure and run node_save().
SQL query, either pure SQL or by custom Drupal (Drush) script. First create your content type, then see the database structure. Then select the body text from the table node_revisions and insert it into your field value column for each node.