Is the Toccata node database upgrade reversible?

No — upgrading a node database to the Toccata pre-release is a one-way operation. Once upgraded, the database cannot be rolled back to an earlier version; the only way to return is a full resync from scratch. For large operators and mining pools, a full resync carries significant time and operational cost. This is why the Kaspa developers recommend testing the pre-release on a limited subset of infrastructure first rather than fully migrating, giving operators a safe path back if issues arise.