Often, a legacy system gets left out of an overall enterprise digital strategy, and issues get addressed on a need 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 is unable to address the business need, you can take a big-bang approach. In the big-bang method, you can build a new system from scratch and shut down the old system. This approach has more risk 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 old and new systems. A phased approach has less risk but takes a long time and may be more expensive as you need to maintain both environments, with additional network and infrastructure bandwidth.
Taking any of these approaches...