How to change textformat of many nodes at once?
In previous instances where this has happened to me, I've just updated the *_format column in the field_revision_* &amp; field_data_* tables, flushed the Drupal cache, and things worked without issue. 

For example, if you're looking to change the nodes' body format for all imported nodes of type my_type from my_format to plain_text, you could execute this query:

UPDATE field_data_body SET body_format = 'plain_text' WHERE entity_type = 'node' AND bundle = 'my_type' AND body_format = 'my_format';
UPDATE field_revision_body SET body_format = 'plain_text' WHERE entity_type = 'node' AND bundle = 'my_type' AND body_format = 'my_format';


NOTE: There's probably a better, programmatic way, than touching the database directly, but I've never seen any adverse effects from this method.