Step 2: Study validity of changes against the new GAS version

At this point, you have patch files containing the customizations you have made. You now need to determine if all theses changes are still valid in the new GAS version.

The kind of customizations that require attention are:

Bug corrections. If your customization provided a work-around for a bug, and that bug is corrected in the new version, that change should be deprecated and discarded.
Feature additions. If your customization provided a feature that was missing in the GAS, but that is now provided by the new GAS, that change should be deprecated and discarded.

All remaining changes need to be migrated to the latest template or snippet files.

When deprecated and valid changes both exist for a template or snippet

If both deprecated and valid changes are present in same template or snippet, you need to remove the deprecated changes from the patch file:

Patch example:

--- /opt/gas/tpl/SetAjax/Image.xhtml 2009-04-09 11:18:31.000000000 +0200
+++ /opt/gas/tpl/MyCustomSetAjax/Image.xhtml 2009-11-19 12:00:51.000000000 +0100
@@ -30,6 +30,8 @@
            > 
                <tbody>
                    <tr>
+                       <!-- MyCustom -->
+                       <p>Good custom...</p>
                        <td valign="top"
                            gwc:define="
                                type switch(substring(value,indexof(value,'.')), 
                                ['.svg','.swf'], [1,2], 0);
                                @@ -71,6 +73,8 @@
                                />
                            </object>
                        </td>
+                       <!-- MyCustom -->
+                       <p>Deprecated custom...</p>
                    </tr>
                </tbody>
            </table>

Simply remove the second change chunk in patch file to obtain:

--- /opt/gas/tpl/SetAjax/Image.xhtml 2009-04-09 11:18:31.000000000 +0200
+++ /opt/gas/tpl/MyCustomSetAjax/Image.xhtml 2009-11-19 12:00:51.000000000 +0100
@@ -30,6 +30,8 @@
            > 
                <tbody>
                    <tr>
+                       <!-- MyCustom -->
+                       <p>Good custom...</p>
                        <td valign="top"
                            gwc:define="
                                type switch(substring(value,indexof(value,'.')), 
                               ['.svg','.swf'], [1,2], 0);