Legacy code Problems for the Enterprise

some people define the Legacy code as the code that works correctly and have been tested and worked with for long years.
the legacy code in many cases need to be ported to accomplish with up-to-date standards and new requiremnets that it is hard or impossible to be implemented in the original legacy code.
A porting may be necessary ,to maintian the same user experience and the quality of system.

Now how to port
I recommend the following approach
but if not be possible