Nano version‑20 ledger schema change: evaluating safe rollback feasibility

Short answer

A rollback to the pre-v20 ledger schema is only safe if the node has not yet committed any v20-format blocks to its local ledger — that is, the rollback point must sit strictly below the first block using the new format. Once a node has processed v20 blocks, reverting the software alone will cause it to reject those blocks on restart, producing a permanent local fork unless the rest of the network reverts the same blocks too. There is no documented automatic rollback path, so treat this as a coordinated, manual operation.

Likely explanation vs. confirmed constraints

Confirmed by your own constraints: the upgrade is forward-only, older software rejects the new block format, and a partial rollback splits the network. The likely explanation for why this feels risky is that Nano's consensus depends on voting weight, not just ledger content — rolled-back nodes that fall behind or reject valid v20 blocks effectively drop out of quorum participation, and if enough weight does this, confirmation stalls or diverges.

Conditions for a consistency-preserving rollback

1. Local condition: the node's ledger contains zero v20-format blocks. If any exist, they must be rolled back to a height below the first v20 block, and every dependent account chain must remain valid after truncation.
2. Network condition: a supermajority of online voting weight (roughly >67%) must either still run pre-v20 software or participate in the same coordinated rollback. If upgraded nodes keep confirming v20 blocks, rolled-back nodes will diverge permanently.
3. Timing condition: the rollback must happen before the v20 block type gains broad confirmation depth; the deeper confirmed v20 blocks are, the less realistic a network-wide revert becomes.

Is dual-write or a version-bit flag enough?

Not by itself. A dual-write mode (storing both representations) or a version-bit flag only prevents divergence if all nodes implement the identical mechanism and enforce rejection of mismatched blocks during the transition window. A version bit does nothing for a node that has already stored v20 blocks and tries to serve them to pre-v20 peers — those peers will reject the blocks and consensus is disrupted anyway. These mechanisms are coordination aids, not safety guarantees.

Verifying funds remain accessible

• Replay the ledger from genesis (or a trusted pre-v20 snapshot) up to the rollback height with the old software and compare the resulting state against a known-good backup or checkpoint.
• Confirm every account chain contains only block types the pre-v20 schema recognizes — in particular, no blocks referencing v20-only fields such as the link-as-account representation — and that all signatures still validate.
• Check that balances match the pre-upgrade snapshot for representative accounts, especially any that received transactions near the upgrade boundary.
• After restart, monitor logs for persistent fork or rejection warnings and confirm peers still accept the node's voting weight.

Practical recommendation

For a single operator, the realistic safe path is: restore from a pre-v20 backup taken before the node processed any v20 blocks, run pre-v20 software, and resync. If the node has already ingested v20 blocks and the network has moved on, rolling back is not feasible without a coordinated network decision — running the upgraded software is the safer choice. One detail that changes this recommendation: has your node actually confirmed any v20-format blocks yet? If not, a straightforward downgrade is viable; if yes, stop and reassess against the network conditions above.

Note: this reflects the general forward-only-upgrade behavior described in your constraints. Verify the exact v20 schema changes and any official rollback guidance against the current Nano node documentation before acting.