Robert Annett wrote a post titled Diagrams for System Evolution where he describes a simple approach to showing how to visually describe changes to a software architecture. In essence, in order to show how a system is to change, he'll draw different versions of the same diagram and use colour-coding to highlight the elements/relationships that will be added, removed or modified.
I've typically used a similar approach for describing as-is and to-be architectures in the past too. It's a technique that works well. Although you can version control diagrams, it's still tricky to diff them using a tool. One solution that addresses this problem is to not create diagrams, but instead create a textual description of your software architecture model that is then subsequently rendered with some tooling. You could do this with an architecture description language (such as Darwin) although I would much rather use my regular programming language instead.
And since the diagrams were created by a model described as Java code, that description can be diff'ed using your regular toolchain.
This perhaps isn't as obvious as Robert's visual approach, and I would likely still highlight the actual differences on diagrams using notation as Robert did too. Creating a textual description of a software architecture model does provide some interesting opportunities though.
Simon is an independent consultant specializing in software architecture, and the author of Software Architecture for Developers (a developer-friendly guide to software architecture, technical leadership and the balance with agility). He’s also the creator of the C4 software architecture model and the founder of Structurizr, which is a collection of open source and commercial tooling to help software teams visualise, document and explore their software architecture.