When Requirements Change: Revise the Design

Why this matters

A compliance rule lands mid-year requiring data residency the platform was never designed for, and teams learn it from an audit finding, not from architecture. Retrofits start in panic: residency bolted onto a design that assumed free movement, at triple the cost of designing it in. Requirement changes announce themselves months early to anyone watching (draft regulations, security reviews, growth plans); the failure is detection, not luck. Watching is a scheduled activity with an owner, not background awareness.

Concepts

Detection has sources and owners. Regulatory drafts, customer security questionnaires, capacity plans, incident trends: each source has a watcher and a review cadence. The watch list is short and written; everything else is noise until it knocks. A changed requirement gets a one-page brief: what changed, what it invalidates in current designs, who is affected, by when. The brief precedes the redesign; redesign without a brief argues about the wrong change.

Revision is a new ADR, not patches on the old. The old decision stays readable (superseded, linked); the new one restates context with the changed requirement, re-runs only the invalidated comparisons (the L71 lab revises under a capacity or security change), and records the migration cost honestly. Patching around expired context preserves the form of the old design while destroying its logic; the result satisfies neither the old requirement nor the new.

Communication moves teams before breakage. Affected teams get the brief, the new decision and the timeline with migration support, not a surprise deprecation. The rollout follows the delivery discipline the program teaches: staged, verified, with a way back. Requirement changes handled this way become routine operations; handled by surprise, they become the year's incidents.

Worked example

A fixture capacity requirement doubles mid-design. The learner writes the change brief (what invalidates: sizing, bottleneck ranking, cost math), supersedes the sizing ADR with revised numbers, and notifies the affected fixture teams with timeline and support. The old and new records read as a clean sequence, not as an argument.

Common wrong move

Quietly stretching the old design past its assumptions. Headroom absorbs the first doubling, then the bottleneck arrives unannounced at the worst moment, and the redesign happens under incident pressure at panic prices. Revise on the brief, not on the outage.