Coexistence Period
The interval during which old and new systems both operate, which is longer, more expensive and more complex than migration plans usually assume.
Every incremental migration has one, and it is routinely treated as a brief transitional inconvenience rather than as a design problem with its own cost.
What must be designed for it:
Data synchronisation between the two systems, in whichever directions are required. Bidirectional synchronisation is substantially harder than one-way and needs a conflict resolution rule decided in advance.
Routing that decides which system handles a given request, by capability, by customer, or by percentage — with the ability to change it quickly.
Reconciliation running continuously, because divergence is silent and compounds.
Duplicate operational cost, since both systems are running, monitored and supported.
Feature freeze or dual implementation on the old system. If the business continues changing it, every change must be made twice — which is the mechanism by which coexistence periods extend indefinitely.
The strategic point: coexistence cost grows with duration, so the programme's economics depend on keeping it short. That argues for migrating in increments that complete rather than starting many in parallel, and for funding the decommissioning explicitly, since a coexistence period that never ends is the most expensive outcome available.