diff --git a/app/code/core/Mage/Sales/Model/Quote/Address.php b/app/code/core/Mage/Sales/Model/Quote/Address.php
index e08dbfe9e99..8cb957abaeb 100644
--- a/app/code/core/Mage/Sales/Model/Quote/Address.php
+++ b/app/code/core/Mage/Sales/Model/Quote/Address.php
@@ -322,7 +322,7 @@ protected function _populateBeforeSaveData()
              * Set same_as_billing to "1" when default shipping address is set as default
              * and it is not equal billing address
              */
-            if (!$this->getId()) {
+            if (!$this->getId() && !$this->hasSameAsBilling()) {
                 $this->setSameAsBilling((int) $this->_isSameAsBilling());
             }
         }
diff --git a/tests/unit/Mage/Sales/Model/Quote/AddressTest.php b/tests/unit/Mage/Sales/Model/Quote/AddressTest.php
new file mode 100644
index 00000000000..b05db049995
--- /dev/null
+++ b/tests/unit/Mage/Sales/Model/Quote/AddressTest.php
@@ -0,0 +1,160 @@
+<?php
+
+/**
+ * @copyright  For copyright and license information, read the COPYING.txt file.
+ * @link       /COPYING.txt
+ * @license    Open Software License (OSL 3.0)
+ * @package    OpenMage_Tests
+ */
+
+declare(strict_types=1);
+
+namespace unit\Mage\Sales\Model\Quote;
+
+use Mage;
+use Mage_Customer_Model_Group;
+use Mage_Sales_Model_Quote;
+use Mage_Sales_Model_Quote_Address;
+use OpenMage\Tests\Unit\OpenMageTest;
+
+final class AddressTest extends OpenMageTest
+{
+    /**
+     * Test that explicitly set same_as_billing flag is preserved for guest orders
+     *
+     * This test validates the fix for the issue where guest order shipping addresses
+     * were being overwritten by billing addresses during order edit.
+     *
+     * @covers Mage_Sales_Model_Quote_Address::_populateBeforeSaveData
+     * @group Model
+     */
+    public function testPreservesExplicitlySetSameAsBillingForGuestOrders(): void
+    {
+        // Create a quote for a guest customer (no customer ID)
+        $quote = new Mage_Sales_Model_Quote();
+        $quote->setCustomerId(null);
+        $quote->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);
+
+        // Create a shipping address with different data than billing
+        $address = new Mage_Sales_Model_Quote_Address();
+        $address->setQuote($quote);
+        $address->setAddressType(Mage_Sales_Model_Quote_Address::TYPE_SHIPPING);
+
+        // Explicitly set same_as_billing to false (0) to indicate different addresses
+        // This simulates what happens during order edit when addresses differ
+        $address->setSameAsBilling(0);
+
+        // Trigger the _populateBeforeSaveData method via _beforeSave
+        // This is normally called when saving the address
+        $reflectionClass = new \ReflectionClass($address);
+        $method = $reflectionClass->getMethod('_populateBeforeSaveData');
+        $method->invoke($address);
+
+        // Assert that the explicitly set value was preserved
+        self::assertSame(
+            0,
+            $address->getSameAsBilling(),
+            'Explicitly set same_as_billing=0 should be preserved for guest orders during order edit',
+        );
+    }
+
+    /**
+     * Test that same_as_billing is set to default for new addresses without explicit value
+     *
+     * @covers Mage_Sales_Model_Quote_Address::_populateBeforeSaveData
+     * @group Model
+     */
+    public function testSetsDefaultSameAsBillingForNewGuestAddressesWithoutExplicitValue(): void
+    {
+        // Create a quote for a guest customer (no customer ID)
+        $quote = new Mage_Sales_Model_Quote();
+        $quote->setCustomerId(null);
+        $quote->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);
+
+        // Create a shipping address WITHOUT explicitly setting same_as_billing
+        $address = new Mage_Sales_Model_Quote_Address();
+        $address->setQuote($quote);
+        $address->setAddressType(Mage_Sales_Model_Quote_Address::TYPE_SHIPPING);
+
+        // DO NOT set same_as_billing - let the default logic handle it
+
+        // Trigger the _populateBeforeSaveData method
+        $reflectionClass = new \ReflectionClass($address);
+        $method = $reflectionClass->getMethod('_populateBeforeSaveData');
+        $method->invoke($address);
+
+        // For guest orders, default behavior should set same_as_billing to 1
+        self::assertSame(
+            1,
+            $address->getSameAsBilling(),
+            'Default same_as_billing=1 should be set for new guest shipping addresses without explicit value',
+        );
+    }
+
+    /**
+     * Test that explicitly set same_as_billing=1 is preserved
+     *
+     * @covers Mage_Sales_Model_Quote_Address::_populateBeforeSaveData
+     * @group Model
+     */
+    public function testPreservesExplicitlySetSameAsBillingTrue(): void
+    {
+        // Create a quote for a guest customer
+        $quote = new Mage_Sales_Model_Quote();
+        $quote->setCustomerId(null);
+        $quote->setCustomerGroupId(Mage_Customer_Model_Group::NOT_LOGGED_IN_ID);
+
+        // Create a shipping address
+        $address = new Mage_Sales_Model_Quote_Address();
+        $address->setQuote($quote);
+        $address->setAddressType(Mage_Sales_Model_Quote_Address::TYPE_SHIPPING);
+
+        // Explicitly set same_as_billing to true (1)
+        $address->setSameAsBilling(1);
+
+        // Trigger the _populateBeforeSaveData method
+        $reflectionClass = new \ReflectionClass($address);
+        $method = $reflectionClass->getMethod('_populateBeforeSaveData');
+        $method->invoke($address);
+
+        // Assert that the explicitly set value was preserved
+        self::assertSame(
+            1,
+            $address->getSameAsBilling(),
+            'Explicitly set same_as_billing=1 should be preserved',
+        );
+    }
+
+    /**
+     * Test that the fix works for registered customers with different addresses
+     *
+     * @covers Mage_Sales_Model_Quote_Address::_populateBeforeSaveData
+     * @group Model
+     */
+    public function testPreservesExplicitlySetSameAsBillingForRegisteredCustomers(): void
+    {
+        // Create a quote for a registered customer
+        $quote = new Mage_Sales_Model_Quote();
+        $quote->setCustomerId(123); // Registered customer
+
+        // Create a shipping address
+        $address = new Mage_Sales_Model_Quote_Address();
+        $address->setQuote($quote);
+        $address->setAddressType(Mage_Sales_Model_Quote_Address::TYPE_SHIPPING);
+
+        // Explicitly set same_as_billing to false
+        $address->setSameAsBilling(0);
+
+        // Trigger the _populateBeforeSaveData method
+        $reflectionClass = new \ReflectionClass($address);
+        $method = $reflectionClass->getMethod('_populateBeforeSaveData');
+        $method->invoke($address);
+
+        // Assert that the explicitly set value was preserved
+        self::assertSame(
+            0,
+            $address->getSameAsBilling(),
+            'Explicitly set same_as_billing=0 should be preserved for registered customers',
+        );
+    }
+}