Whether to enroll contacts who already meet the trigger conditions. When omitted or false, only future trigger events are processed.

When enrollExisting=true:

• Event-based triggers (e.g. "placed order", "started checkout"): contacts whose trigger event occurred within the first delay block's time window are enrolled. They receive messages based on their original event time, not when the workflow was enabled (e.g. 30-day delay, event 20 days ago → message in 10 days). The workflow must start with a delay block — otherwise the request is rejected with 409 enroll-existing-not-applicable.
• "Entered segment" trigger: all contacts currently in the trigger's segments are enrolled immediately. Takes effect only on the first enable; re-enabling with enrollExisting=true is rejected with 409 enroll-existing-not-applicable. Copy the workflow if a fresh enrollment is needed.
• Birthday, "Product back in stock", and custom virtual events: not supported; enrollExisting=true is rejected with 409 enroll-existing-not-applicable.