Thanks for sharing your findings.
When using replacement behaviour, the subscription in the API will change at the time the merchant accepts the plan change since it’s not possible to have 2 subscriptions at the same time. The replacement is specifically for when the new billing is applied.
If it’s deferred to the next billing cycle, the merchant will pay the new charge at the start of the next billing cycle.
If the replacement is immediate, then the merchant will receive the standard proration or deferral behaviour.