Mass post format changer?

• Back-up your db

• Add this code to your functions.php file from your theme:

  $all_posts = get_posts(array(
    'numberposts' => -1,
    'post_type'   => 'post',
  ));
  
  foreach($all_posts as $p)
    set_post_format($p->ID, 'standard');
  

• Refresh
• Remove the code

Or with a SQL command:

UPDATE `term_relationships`
  SET term_taxonomy_id = 'standard_post_format_id'
  WHERE term_taxonomy_id = 'gallery_post_format_id'

Change standard_post_format_id and gallery_post_format_id with the IDs matching post format names from the term_taxonomy/terms tables