Modernizing and reengineering on the legacy systems
is an essential need in software industries. The replacement of legacy
software systems is one of the most effective parts in process of modernizing
software systems. In this study, a useful method for replacement of legacy
large systems is proposed. In this method, the incremental approach is
used to create interface and control layers on the corresponding sectors
in two systems. With considering a new system in under-controlled condition
and testing with real data and finding possible errors, replacement process
faces with low risk. In comparison of proposed method with other traditional
methods, two main differences are considered. In proposed method, data
bases and resources of new system shall be different with legacy system.
Second difference is finding errors to make final systems ready to decrease
the risks and increase reliability.