How can I change the Add to Cart button text on specific product forms?
Take help of hook_form_alter. Get the ID of the form for the product display where you are trying to change the text &amp; do something like this --

function MYMODULE_form_alter(&amp;$form, &amp;$form_state, $form_id){//replace your module name with MYMODULE
  if(strpos($form_id, 'commerce-cart-add-to-cart-form-65') !== false){//my form ID is "commerce-cart-add-to-cart-form-65", replace it with your form ID.
    $form['submit']['#value'] = t('Check Out');//Put the text which you want to make appear instead of "Add to cart".
  }
}


I also have several product display with custom text for "Add to cart". It took some time for me figure out the easy way. Have Fun :)