overriding ubercart - add to cart button
In D7, below is my solution for different product classes.

/* Overrides the Add to Cart form text*/
if ( !empty($form['nid']) ) {
  $node = $form['nid']['#value'];
}
else {
  $node = 0;
}
if (($form_id == 'uc_product_add_to_cart_form_'.$node) and ($form['node']['#value']-&gt;type =='uc_recurring_subscription')){
   $form['actions']['submit']['#value'] = t('Subscribe');     
}

if (($form_id == 'uc_product_add_to_cart_form_'.$node) and ($form['node']['#value']-&gt;type =='credit')){
   $form['actions']['submit']['#value'] = t('Buy');     
}


For regular product or just one product class, this should work.

/* Overrides the Add to Cart form text*/
if ( !empty($form['nid']) ) {
  $node = $form['nid']['#value'];
}
else {
  $node = 0;
}
if (($form_id == 'uc_product_add_to_cart_form_'.$node){
   $form['actions']['submit']['#value'] = t('Buy now');     
}