How to change boostrap seach button class from btn-default to btn-primary?
Copy /sites/all/themes/bootstrap/theme/bootstrap/bootstrap-search-form-wrapper.func.php

to 

/sites/all/themes/my-bootstrap-subtheme/templates

and edit that file at line 14:

 $output .= '&lt;button type="submit" class="btn btn-default"&gt;';


to:

 $output .= '&lt;button type="submit" class="btn btn-primary"&gt;';


This might be helpful:
https://drupal.org/node/225125