# The format upgrade

- A new version of wanderbooks can keep the books in a newer journal format than the version that wrote them. Books an older version wrote wait for a format upgrade: books.start says so (format_upgrade) and puts it first among its next steps, in every session until it is done.
- Until then they work on their old format: everything that needs nothing new works as before, and whatever would need the new format is refused with format_upgrade_pending, changing nothing. Never retry it, work around it, or post it another way.
- An invoice's or bill's number, payment terms, seller and bill-to, references and memo, a line's account, quantity and unit price, the copy of an invoice kept exactly as it was issued, a party's email and default terms, and the invoicing and bills settings all wait for the documents_v2 upgrade. A document that names none of them is written exactly as before and is never refused; the work that needs one waits, and the books say so.
- Only the owner can agree to the upgrade, with the owner passphrase: in the app, which asks each time it opens the books, or with `wanderbooks books upgrade ID` in a terminal. No tool can, and you never ask for, take or type the passphrase. Tell the human, in plain words: the books wait for their go-ahead, what was refused, and that saying no keeps the books working as they are.
- Before it upgrades, the server makes a backup of the books, their vault and keys included, and proves it whole; a backup that can't be made or proven upgrades nothing. Then the books move to the new format in one step, on the record.
- After the upgrade an older version of wanderbooks can't open these books; the backup made first keeps them as they were. Going back is the owner's choice, never yours.
- This upgrade is `documents_v2`: invoice numbers, terms and line accounts (S4), sent records (S5), payments that settle several documents with early-payment discounts and a processor's fee (S6), and write-offs (S7). Until it is given, those writes are refused and everything else works as before.