When a new version of the software is released, there will inevitably be changes to the default templates in which would need to be replicated in your own custom themes. On an upgrade, only unedited templates would be upgraded automatically, and therefore this would be for yourself to upgrade any templates which cannot be upgraded.
There are a couple of tools available for you in order to achieve this. Each of which is explained below.
The theme differences tool is an external tool which can be found in the following location on our site
This tool will allow you to see the differences between any 2 versions of the 4.x software, from 4.0.0 right up to the current version.
Once you have selected the versions to compare, you will then be shown all changes between templates on the 2 versions in which you selected in a similar format to the below. Red showing what has been removed, and the green showing what was added
Comparing within the admin CP
Whilst within a template in your custom theme, you can compare that to the default, unedited version of the same template directly from the admin CP.
First of all, you would take a look to see which items within your custom theme have been modified. You can do this using the selection menu at the top of the template list, to select only modified templates. This will enable you to quickly find all templates that you may need to look at after an upgrade.
Once you have found and opened a template that you are working on, you will see a tools button in the top right of the screen. Clicking on this will show an option for you show the default.
If you simply need to change the template to be that of the default template, you can use the Revert button, also shown in the image below, to achieve this.
Once you have selected this, you will see the default template is placed alongside, with any changes to the default indicated, as shown below. Note that you can edit directly on the right hand side to make any changes needed.