diff --git a/rust/README.md b/rust/README.md
index 4d29b53..7133d2b 100644
--- a/rust/README.md
+++ b/rust/README.md
@@ -25,7 +25,7 @@ ping-collector fill-gaps <days|all> # re-fetch RID reports with holes
 ping-collector refetch FROM [TO]    # re-fetch the RID reports of FROM..=TO (YYYY-MM-DD) even
                                     # without holes, through the spike screening: corrects
                                     # readings RID revised, deletes stored spikes; prints the
-                                    # catch-up-forecasts command when readings changed
+                                    # catch-up-forecasts command when stored levels changed
 ping-collector check-db             # connect, create missing tables, row counts
 ping-collector forecast [--json] [--save] [--now T] [--rain-file F]
                                     # forecast from the database and print it
diff --git a/rust/collector/src/db_tests.rs b/rust/collector/src/db_tests.rs
index 416dd97..57231e0 100644
--- a/rust/collector/src/db_tests.rs
+++ b/rust/collector/src/db_tests.rs
@@ -272,8 +272,8 @@ async fn spikes_are_screened_and_refetch_repairs_the_store() {
     let s = rid::refetch(&src, &db.pool, &stations, day, day)
         .await
         .unwrap();
-    assert_eq!((s.saved, s.spikes, s.deleted), (1, 1, 1));
-    assert_eq!(s.first_change, Some(t(2026, 10, 24, 23, 0)));
+    assert_eq!((s.saved, s.spikes, s.deleted, s.changed), (1, 1, 1, 2));
+    assert_eq!(s.first_change, Some(t(2026, 10, 25, 0, 0)));
     assert_eq!(
         p1_levels(&db.pool).await,
         vec![
@@ -291,8 +291,17 @@ async fn spikes_are_screened_and_refetch_repairs_the_store() {
         .await
         .unwrap();
     assert_eq!((s.saved, s.spikes, s.deleted, s.changed), (2, 0, 0, 1));
-    assert_eq!(s.first_change, Some(t(2026, 10, 25, 0, 0)));
+    assert_eq!(s.first_change, None); // a new hour, nothing to redo
     assert_eq!(p1_levels(&db.pool).await.len(), 3);
+    // RID revises a level: the forecasts from that hour on need a redo
+    src.0
+        .lock()
+        .unwrap()
+        .insert(day, vec![(23, 1.05), (24, 1.06)]);
+    let s = rid::refetch(&src, &db.pool, &stations, day, day)
+        .await
+        .unwrap();
+    assert_eq!(s.first_change, Some(t(2026, 10, 24, 23, 0)));
 
     // a live cycle: the newest hour jumps and waits for the next one
     let today = d(2026, 10, 25);
@@ -315,7 +324,7 @@ async fn spikes_are_screened_and_refetch_repairs_the_store() {
     let o = rid::run_cycle(&src, &db.pool, &stations, t(2026, 10, 25, 3, 10)).await;
     assert_eq!((o.fetched, o.saved), (5, 4));
     let levels: Vec<f64> = p1_levels(&db.pool).await.iter().map(|r| r.1).collect();
-    assert_eq!(levels, vec![1.04, 1.06, 1.07, 1.1]);
+    assert_eq!(levels, vec![1.05, 1.06, 1.07, 1.1]);
 
     // the forecasts issued from the changed hours can be redone
     let mut f = forecast_row("P.1", 24, 0.9);
diff --git a/rust/collector/src/main.rs b/rust/collector/src/main.rs
index 89f4f56..e5eed96 100644
--- a/rust/collector/src/main.rs
+++ b/rust/collector/src/main.rs
@@ -255,7 +255,7 @@ async fn run(cli: Cli) -> Result<()> {
                 let hours = (util::bangkok_now() - t).num_hours() + 2;
                 if hours <= 24 * 60 {
                     println!(
-                        "forecasts issued since {t} came from other readings; recompute them with\n  ping-collector catch-up-forecasts --hours {hours} --redo-from {}",
+                        "stored levels changed from {t} on: the forecasts issued since came from wrong levels; recompute them with\n  ping-collector catch-up-forecasts --hours {hours} --redo-from {}",
                         t.format("%Y-%m-%dT%H:%M")
                     );
                 }
diff --git a/rust/collector/src/rid.rs b/rust/collector/src/rid.rs
index 0bb0372..c79b83b 100644
--- a/rust/collector/src/rid.rs
+++ b/rust/collector/src/rid.rs
@@ -251,8 +251,10 @@ pub struct StoreOutcome {
     pub deleted: usize,
     /// Saved readings that were new or differ from the stored ones, plus deletions.
     pub changed: usize,
-    /// The earliest hour whose stored data changed (forecasts from then on
-    /// were computed from other data).
+    /// The earliest hour whose stored level changed or was deleted: the
+    /// forecasts issued from then on came from a wrong level. (New hours and
+    /// added discharges do not count: the catch-up forecasts new hours, and
+    /// a forecast issued without the discharge is still what was issued.)
     pub first_change: Option<NaiveDateTime>,
 }
 
@@ -322,12 +324,23 @@ pub async fn store(
         .map(|r| r.timestamp)
         .collect();
     changed.extend(s.delete.iter().map(|(t, _)| *t));
+    let level_changed = s
+        .keep
+        .iter()
+        .filter(|r| {
+            before
+                .get(&(r.timestamp, r.station_num))
+                .is_some_and(|(l, _)| differs(*l, r.water_level, 3))
+        })
+        .map(|r| r.timestamp)
+        .chain(s.delete.iter().map(|(t, _)| *t))
+        .min();
     out.saved = db::save_rid(pool, &s.keep, stations).await?;
     out.deleted = db::delete_rid_readings(pool, &s.delete).await?;
     out.held = s.held.len();
     out.spikes = s.spikes.len();
     out.changed = changed.len();
-    out.first_change = changed.into_iter().min();
+    out.first_change = level_changed;
     Ok(out)
 }