There are different approaches to upgrading existing mainframe applications to Enterprise Developer in order to modernize them.
Starting over and rewriting your application in Enterprise Developer is not recommended as this means more costs, the need to do more testing and, potentially, incurs more risks.
You can keep the existing application intact and only create new front ends in Enterprise Developer that will interact with the application.
Or, and this might be the best approach and lower risk one in most situations, is to upgrade your applications and processes using incremental steps. In order to reduce the maintenance backlog and ensure that the applications have the flexibility and agility that the business needs. Using this approach: