SYSE.34:2 - Problem

“Add fields, copy data, switch readers” leaves important work unspecified. Old writers may continue changing the original representation after a copy. Two new authorities can disagree. A retried backfill may overwrite a legitimate later write with a cached earlier value.

Returning the old executable is not enough when the new software has removed information or written values the old software cannot interpret. Even a successful database restore can omit later business events or repeat external effects. Recovery needs a real state boundary, not only a rollback button.