The project needs an executive champion. It is not an IT project. Manage the project with proper ownership of tasks, planning and control. Not all data is dynamic but some is. In a staged implementation process this needs to be taken into account.
Parallel running the new and old system with dynamic data is extremely difficult.
Parallel running reveals as many problems in the old system as in the new system. Software problems are a useful excuse for project delays but they are rarely on the critical path of delivering business benefit. Business Issues Identify the data owners. Operational responsibilities for the data including executive ownership need to be defined and agreed. Computer salesmen do not provide solutions.