Roll out & decommission

With parity proven, you don't flip the whole site at once; you advance like a careful tide, one section at a time, always keeping a way back. Mixed implementation is supported across pages (some on Web SDK, some still on AppMeasurement) but never on the same page, so you migrate section by section, watch each, and only then move on. The final act is decommissioning: turning off the legacy extensions and removing the old scripts until AppMeasurement is gone. Keep your previous Launch library versions, that is your rollback, and you want it documented before you need it, not during an incident.