The cryptography is not the upgrade. Merkle commitment, publicly anchored roots, inclusion and consistency proofs, and offline verification are on every plan — including the free one. An audit log you have to pay to verify isn't an audit log.
The whole proof, at hobby volume. No card, no trial clock.
Production volume, metered overages, and the controls a team needs.
Events are metered as they are sealed — one event, one unit, at the moment it is committed to the tree. Nothing else is metered: storage is shown to you as a live figure but never billed, since it only re-measures the events you have already paid for.
Overages are billed per unit rather than as a forced plan jump.