Setting #element_validate in a form (not form alter) removes original #element validation
You can create validate callback for the entire form and validate the element in that callback.$form['#validate'][] = 'new_validate'; 

Also separate validate callbacks can be attached to 'submit' ('preview', 'cancel' etc) buttons within the form. And the element can be validated there as well.

$form['submit_button']['#validate'][] = 'new_validate';