Strangler (Fig) Pattern

Web_App

_

Web_App

The cutover

Web_App

_

Strangler

Middleware

Web_App

_

Strangler

Middleware

New_App

Web_App

_

Strangler

Middleware

New_App

Web_App

_

Strangler

Middleware

New_App

Web_App

_

Strangler

Middleware

New_App

Route

Route

x

Web_App

_

New_App

Route

Route

x

Proxy

You could migrate all functionality from an old technology solution to new one in a series of releases that focused on nothing else. Some companies will do that as there is a lot of sense to getting your house in order before doing anything else.

https://paulhammant.com/2013/07/14/legacy-application-strangulation-case-studies/