How to delete node revisions
You can use the following code.

db_query('DELETE FROM {node_revisions} nr WHERE nr.vid NOT IN (SELECT n.vid FROM {node} n)');


You don't want to delete all the revisions: You want to delete all but the current revision.

Executing only that query is going to leave data about deleted revisions in other database tables, as modules that save information about a revisions are not notified a revision is going to be deleted. I am thinking of the CCK module, for example, which saves information about the node fields in its own database tables. The CCK module executes the following code, when a revision is deleted.

/**
 * Implementation of hook_nodeapi 'delete_revision' op.
 *
 * Delete node type fields for a revision.
 */
function content_delete_revision(&amp;$node) {
  _content_field_invoke('delete revision', $node);
  _content_field_invoke_default('delete revision', $node);
  cache_clear_all('content:'. $node-&gt;nid .':'. $node-&gt;vid, content_cache_tablename());
}


A more complete code would be similar to the following one.

$query = db_query('SELECT nr.nid, nr.vid FROM {node_revisions} nr WHERE nr.vid NOT IN (SELECT n.vid FROM {node} n)');

while ($revision = db_fetch_object($query)) {
  if ($node = node_load($revision-&gt;nid, $revision-&gt;vid)) {
    node_invoke_nodeapi($node, 'delete revision');
  }
}

db_query('DELETE FROM {node_revisions} nr WHERE nr.vid NOT IN (SELECT n.vid FROM {node} n)');


Looking at the code of node_revision_delete_confirm_submit(), I don't see anything else that needs to be done.

The new code I wrote could take much time to be executed, depending on the number of nodes, and revisions. This means the code should be executed:


In an implementation of hook_cron()
As a batch operation
In a PHP file that bootstraps Drupal, and increases the execution time with @set_time_limit(240)
With Drush


Considering that the code is only necessary for the migration, I would avoid the first two options, which would require you to write a module you need just in this very specific case. If you have more sites, and all of them need to execute the same code, option 3 is always possible, and that is what is done from Drupal with cron.php.
About option 4, I would first check Drush doesn't have already commands that could be helpful in this case. I would not reinvent the wheel, when it is not strictly necessary. I would also check how to write Drush Shell Scripts.