The operational problem
A subscription integration can grow across Flows, invocable actions, asynchronous jobs, webhook handlers and older branches. The business still needs customer, user and entitlement processing to work, but diagnosing a failure becomes difficult.
A replacement needs to preserve the useful capabilities without copying every historical implementation choice.
Design around business capabilities
Build a capability map covering each input, customer-facing result and operator action. Use it to define what the replacement must preserve and how each behaviour will be tested.
Give authentication, transport and external API handling clear service boundaries. Keep Salesforce orchestration and operator actions in appropriate Flow or Lightning components, with webhook handling passing into the supported processing layer.
Prove the replacement before cutover
Validate the capabilities in non-production against the external service’s sandbox. Include failed requests, retries, duplicate events and the support actions needed to diagnose and repair a problem.
Limit any coexistence of old and new routes to the agreed transition. Acceptance should cover the required endpoints, business behaviour, error visibility and removal of replaced logic.