When the evidence from closed-won and retention disagrees with the definition — not when a quarter is soft. Re-score on defined triggers: a new product or use case, a segment that starts retaining materially better or worse, or a shift in where deals actually originate.
Changing ICP has downstream cost: territories, routing rules, scoring, outbound lists and quota all reference it. Treat it as a versioned decision with a date, so reporting can distinguish a performance change from a definition change.
Every claim above comes from work LeanScale published. These are the sources.