In the first step, go through your existing (and hopefully well documented) business processes and find out which process domains are influenced by the change. The interesting point at this level is the business process landscape consisting of the business roles assigned to the services, the services themselves and the relationships between them.
In the base architecture analysis it is very important that you do not stop at the process level but also identify your business services strongly related to your business processes, the application services implementing your business processes and at least the application components providing application services.
On a master level you also analyse two steps further in detail i.e. the real software components on which applications run and the infrastructure components needed to run your software.
It is very important to use a standard description language for modelling your process landscape and enterprise architecture. We suggest ArchiMate for the landscape and BPMN for the process details. These standards are widely used and fulfill all requirements of an enterprise architecture and business process description language. Furthermore, you find a great number of tools supporting these description languages.
The following figure illustrates the relevant processes, services and business roles as part of the enterprise architecture of our example enterprise. The infrastructure layer is ommitted in this illustration just for simplifying the diagram but should never be neglected in a real analysis.
When you have done this, you have analyzed your base enterprise architecture in all domains influenced by your business model change as shown in the figure.