[qualiteam]

It depends on how you want the field to behave.

If you search the source code for "state_id" word, you'll get the idea of how many files must be edited to make the field work exactly like the state field does at the moment.

However, you may try to replace the input field with a select box without changing the database structure. This will require a custom module. If you decide to try this, the files you should look into are skins/customer/address/form/body.twig and classes/XLite/View/Address.php. I believe you will have to "decorate" \XLite\View\Address::getSchemaFields() method and make it return your custom field class (extended from \XLite\View\FormField\Select\ASelect).