Hi,
When the checkout legacy is activated, you'll have only one address form on the page, never two. So you won't have a problem in CSS or in JS to target a field of the form, if that's what you want.
As we don't know what you want to do, I thought that this would be better.
A str_replace is really easy:
echo str_replace('STRING TO REPLACE','REPLACEMENT STRING',$this->fieldsClass->display(
$oneExtraField,
$this->$type->$fieldName,
'data['.$type.']['.$fieldName.']',
false,
' '.$onWhat.'="hikashopToggleFields(this.value,\''.$fieldName.'\',\''.$type.'\',0);"'
));
Note that the exact view where to do the change will vary based on how your checkout is configured (the address selector, etc) and whether the "checkout legacy" option is activated or not. It might be one of the files of the address view, or one of the files with the text "address" in it in the view "checkout".