I noticed that the ACP is warning me about a few files that have been modified.  Namely Invoice.php

How do I know what exactly has changed in this file?

Thank you 🙂

You’ll have to compare the original files ( fresh download from the client area)  with your customized files 

If you will be needing to check for changes in each release, I suggest using a versioning tool like github. Just make sure you use a private repo.