Strangler Fig
also called Strangler Pattern
Replacing a legacy system incrementally by routing individual capabilities to new implementations behind a facade, until nothing routes to the old system.
A facade — usually a proxy, gateway or router — sits in front of the legacy system. Initially it forwards everything. One capability at a time is reimplemented and the facade is reconfigured to route that capability to the new implementation. When the last route moves, the old system is switched off.
The value is that every step is small, independently verifiable and independently reversible, so the migration never depends on a single cutover event that either works or ruins a weekend.
The two things that sink it: shared data that both systems write, which reintroduces the coupling you were trying to remove, and the facade becoming permanent because the last 20% of capabilities are the awkward ones nobody funds. Name the end date and the last route in the plan.