The question is too broad. It will depend on your situation and resources. If the code has been stable and secure for 15 years with a bit of maintenance? Why wo

The question is too broad. It will depend on your situation and resources. If the code has been stable and secure for 15 years with a bit of maintenance? Why would you replace it? If you are adding new features or the code has become hard to maintain? The sure refactor and rewrite away. There is something to be said for using timetested code, but you should never turn it into a religion and remain practical and open to revising it if good reasons come along.