How to bulk update text filter format on fields in drupal 7
Turns out this was solved in a different question, the answer is below:

Try this way, by making a loop for all nodes of certain type:
$node = node_load(nid);
$node-&gt;body[$node-&gt;language][0]['format'] = 'full_html'; // plain_text
node_save($node);


Kudos to Ek Kosmos for sorting this out.