Hi Niks, legacy conversion projects are always a challenge. The difficulty is understanding the full set of functionality that the business actually uses today.
I have included the models that correspond to these. You can read more about these models on our blog
requirements.seilevel.com/blog
or in our book we have a section which discusses which models to use for a system replacement project.
http://www.amazon.com/Visual-Models-Software-Requirements-Beatty/dp/0735667721/ref=sr_1_1?ie=UTF8&qid=1343069427&sr=8-1&keywords=visual+models
We have found the following general steps works well:
0) Business objectives model to have a shared understanding of the value the replacement system is supposed to create
1) Identify the business processes (Process Flow)
2) Identify the metrics associated with the business processes (KPI Model) so that you understand the level of throughput the business needs to have
3) Assign value to the processes based on the business throughput
4) CRITICAL STEP -Organize requirements by mapping them to the process flows (Requirements Mapping Matrix). Each process step will map to 3-4 requirements. This will help to ensure that you have a complete set of requirements
5) Map business rules to the requirements - each requirement can map to many business rules (but generally less than 10).
6) Create screen models that support the requirements (Display Action Response Model)
In addition you will probably need a few additional models like
1) ecosystem map to show all the interfaces to other systems
2) report tables - to show all the reports and decisions that the stakeholders use
3) Business data diagram to show the relationships of the data
4) data dictionary to detail out the data
5) org chart to make sure you fully understand the user population you need to work with
Comparing this list of models against what they have already delivered in the BRD