Change entity bundle on update?
That seems dangerous to me - a node's bundle is fundamental to its identity, and has repercussions well beyond the node's data itself. You're on Drupal 7, presumably? Simple way to test would be a script like:

$node = node_load(5);
$node-&gt;type = 'other_type';
node_save($node);


If it breaks there, it'll break with migration too. And if it works there, it should work with migration.