Defining a strategy for system modernization
Often, a legacy system gets left out of an enterprise's overall digital strategy, and issues get addressed on an as-needed basis. Taking a reactive approach holds back organizations from executing overall system modernization and benefits.
If your legacy system has serious business challenges, such as security and compliance issues, or cannot address the business need, you can take a big-bang approach. In the big-bang method, you build a new system from scratch and shut down the old system. This approach is risky but addresses a business need that can be mitigated from the existing legacy system.
The other approach you can take is a phased approach, where you upgrade one module at a time and keep running both the old and the new systems. A phased approach is less risky but takes a long time and may be more expensive as you need to maintain both environments, with the concomitant increased network and infrastructure bandwidth...