Update that variable name that's been bugging me.

authorizenetwebform.admin.inc

@@ -10,7 +10,7 @@
 function authorizenetwebform_admin_settings() {
   $form = array();
 
-  $form['authorizenetwebform_url'] = array(
+  $form['authorizenetwebform_mode'] = array(
     '#type' => 'radios',
     '#title' => t('Transaction mode'),
     '#description' => t('Unless you are using a sandbox account, this should be set to Live.'),
@@ -18,11 +18,11 @@ function authorizenetwebform_admin_settings() {
       'main' => t('Live account'),
       'test' => t('Test (or Sandbox) account'),
     ),
-    '#default_value' => variable_get('authorizenetwebform_url', 'main'),
+    '#default_value' => variable_get('authorizenetwebform_mode', 'main'),
   );
   // Add description text for URL choices.
-  $form['authorizenetwebform_url']['test']['#description'] = t('Posts to https://test.authorize.net/gateway/transact.dll');
-  $form['authorizenetwebform_url']['main']['#description'] = t('Posts to https://secure.authorize.net/gateway/transact.dll');
+  $form['authorizenetwebform_mode']['test']['#description'] = t('Posts to https://test.authorize.net/gateway/transact.dll');
+  $form['authorizenetwebform_mode']['main']['#description'] = t('Posts to https://secure.authorize.net/gateway/transact.dll');
 
   $form['login'] = array(
     '#type' => 'fieldset',

authorizenetwebform.install

 <?php
-
 /**
  * @file
  * Authorize.Net Webform Module Installation File
@@ -41,9 +40,18 @@ function authorizenetwebform_schema() {
  * Implements hook_uninstall().
  */
 function authorizenetwebform_uninstall() {
-  variable_del("authorizenetwebform_login");
-  variable_del("authorizenetwebform_transaction");
-  variable_del("authorizenetwebform_custom_fields");
-  variable_del("authorizenetwebform_url");
+  variable_del('authorizenetwebform_login');
+  variable_del('authorizenetwebform_transaction');
+  variable_del('authorizenetwebform_custom_fields');
+  variable_del('authorizenetwebform_url');
   variable_del('authorizenetwebform_client_key');
 }
+
+/**
+ * Implements hook_update_N. Changes a variable name.
+ */
+function authorizenetwebform_update_7000() {
+  $mode = variable_get('authorizenetwebform_url', 'main');
+  variable_set('authorizenetwebform_mode', $mode);
+  variable_delete('authorizenetwebform_url');
+}

authorizenetwebform.module

@@ -156,7 +156,7 @@ function authorizenetwebform_form_webform_client_form_alter(&$form, &$form_state
 
   // Add accept.js.
   $js_url = 'https://js.authorize.net/v1/Accept.js';
-  if (variable_get('authorizenetwebform_url', 'main') == 'test') {
+  if (variable_get('authorizenetwebform_mode', 'main') == 'test') {
     $js_url = 'https://jstest.authorize.net/v1/Accept.js';
   }
 
@@ -377,7 +377,7 @@ function authorizenetwebform_webform_client_form_submit($form, &$form_state) {
 
   // Set the post URL.
   $post_url = 'https://api.authorize.net/xml/v1/request.api';
-  if (variable_get('authorizenetwebform_url', 'main') == 'test') {
+  if (variable_get('authorizenetwebform_mode', 'main') == 'test') {
     $post_url = 'https://apitest.authorize.net/xml/v1/request.api';
   }