Just copy the contents of any outdated template to notepad.
Revert the template.
Copy the contents of the updated template to notepad.
Do a comparison and merge your changes back in to the new template.
Ideally you should keep a list or file with all your edits, which makes it easier to refer to them during an upgrade.
What are outdated templates and why do I need to revert them? Can't I just continue using the old ones?
If a template is changed between releases, i.e. from 1.0.0 to 1.0.1, and you have manually edited it, then it will be marked as outdated. This is done because the software doesn't automatically overwrite those templates, thereby causing certain functionality which you have added to be lost. It is an indication that the core code has changed and you need to take action. The software will continue to function with these outdated templates but you may be missing important updates and bug fixes so it is recommended to revert and reapply any edits.