How to change documents to another category painlessly
Try Node Convert, it allows you to change one content type to another:


  Node Convert adds a menu tab "Convert" on the node view page, i.e.
  node/1/convert, which gives the ability to convert the node from its
  current node type to another node type. The module provides the means
  to transfer all the field values of the current node type into fields
  on the destination node type, or discard them.


Then you can use Views Bulk Operations to do it in batches.