Strangler pattern, a powerful simple concept to do refactoring

An alternative route is to gradually create a new system around the edges of the old, letting it grow slowly over several years until the old system is strangled. The most important reason to consider a strangler application over a cut-over rewrite is reduced risk. There’s another important idea here – when designing a new application you should design it in such a way as to make it easier for it to be strangled in the future.

Source: www.martinfowler.com