diff --git a/lib/persist/conformance.sh b/lib/persist/conformance.sh
index e25b865b..d7ebd032 100755
--- a/lib/persist/conformance.sh
+++ b/lib/persist/conformance.sh
@@ -13,7 +13,7 @@ if [ ! -x "$SX_SERVER" ]; then
   exit 1
 fi
 
-SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch recovery)
+SUITES=(event log kv project subscribe concurrency snapshot compaction durable blob view cas catalog query batch upcast recovery)
 
 OUT_JSON="lib/persist/scoreboard.json"
 OUT_MD="lib/persist/scoreboard.md"
@@ -41,6 +41,7 @@ run_suite() {
 (load "lib/persist/catalog.sx")
 (load "lib/persist/query.sx")
 (load "lib/persist/batch.sx")
+(load "lib/persist/upcast.sx")
 (load "lib/persist/subscribe.sx")
 (load "lib/persist/api.sx")
 (epoch 2)
diff --git a/lib/persist/scoreboard.json b/lib/persist/scoreboard.json
index 6c07cf9d..d236f68c 100644
--- a/lib/persist/scoreboard.json
+++ b/lib/persist/scoreboard.json
@@ -15,9 +15,10 @@
     "catalog": {"pass": 10, "fail": 0},
     "query": {"pass": 9, "fail": 0},
     "batch": {"pass": 10, "fail": 0},
+    "upcast": {"pass": 9, "fail": 0},
     "recovery": {"pass": 6, "fail": 0}
   },
-  "total_pass": 162,
+  "total_pass": 171,
   "total_fail": 0,
-  "total": 162
+  "total": 171
 }
diff --git a/lib/persist/scoreboard.md b/lib/persist/scoreboard.md
index 5fa603da..88f3b77b 100644
--- a/lib/persist/scoreboard.md
+++ b/lib/persist/scoreboard.md
@@ -19,5 +19,6 @@ _Generated by `lib/persist/conformance.sh`_
 | catalog | 10 | 0 | 10 |
 | query | 9 | 0 | 9 |
 | batch | 10 | 0 | 10 |
+| upcast | 9 | 0 | 9 |
 | recovery | 6 | 0 | 6 |
-| **Total** | **162** | **0** | **162** |
+| **Total** | **171** | **0** | **171** |
diff --git a/lib/persist/tests/upcast.sx b/lib/persist/tests/upcast.sx
new file mode 100644
index 00000000..19295f52
--- /dev/null
+++ b/lib/persist/tests/upcast.sx
@@ -0,0 +1,115 @@
+; Extension — event schema evolution via upcasters.
+
+; v1 "placed" events had {:total N}; v2 wants {:amount N :currency "GBP"}.
+(define up-placed (fn (e) (persist/upcast-data e {:amount (get (persist/event-data e) :total) :currency "GBP"})))
+
+(persist-test
+  "unregistered type passes through unchanged"
+  (let
+    ((reg (persist/upcasters)))
+    (persist/event-data
+      (persist/upcast
+        reg
+        (persist/event "s" 1 "other" 0 {:x 1}))))
+  {:x 1})
+(persist-test
+  "registered upcaster lifts an old event"
+  (let
+    ((reg (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (get
+      (persist/event-data
+        (persist/upcast
+          reg
+          (persist/event "s" 1 "placed" 0 {:total 50})))
+      :amount))
+  50)
+(persist-test
+  "upcaster adds the new field"
+  (let
+    ((reg (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (get
+      (persist/event-data
+        (persist/upcast
+          reg
+          (persist/event "s" 1 "placed" 0 {:total 50})))
+      :currency))
+  "GBP")
+(persist-test
+  "upcast preserves stream/seq/type/at"
+  (let
+    ((reg (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (let
+      ((e (persist/upcast reg (persist/event "orders" 7 "placed" 99 {:total 1}))))
+      (list
+        (persist/event-seq e)
+        (persist/event-at e)
+        (persist/event-type e))))
+  (list 7 99 "placed"))
+(persist-test
+  "registry is immutable — register returns a new dict"
+  (let
+    ((r0 (persist/upcasters)))
+    (begin
+      (persist/register-upcaster r0 "placed" up-placed)
+      (has-key? r0 "placed")))
+  false)
+(persist-test
+  "read-upcast lifts every event in a stream"
+  (let
+    ((b (persist/open))
+      (reg
+        (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (begin
+      (persist/append b "orders" "placed" 0 {:total 10})
+      (persist/append b "orders" "placed" 0 {:total 20})
+      (let
+        ((es (persist/read-upcast b "orders" reg)))
+        (list
+          (get (persist/event-data (nth es 0)) :amount)
+          (get (persist/event-data (nth es 1)) :amount)))))
+  (list 10 20))
+(persist-test
+  "project-upcast folds over the current shape"
+  (let
+    ((b (persist/open))
+      (reg
+        (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (begin
+      (persist/append b "orders" "placed" 0 {:total 10})
+      (persist/append b "orders" "placed" 0 {:total 20})
+      (persist/project-upcast
+        b
+        "orders"
+        reg
+        (fn (acc e) (+ acc (get (persist/event-data e) :amount)))
+        0)))
+  30)
+(persist-test
+  "mixed old and new events fold uniformly"
+  (let
+    ((b (persist/open))
+      (reg
+        (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (begin
+      (persist/append b "orders" "placed" 0 {:total 5})
+      (persist/append b "orders" "placed" 0 {:total 7 :amount 7})
+      (persist/project-upcast
+        b
+        "orders"
+        reg
+        (fn (acc e) (+ acc (get (persist/event-data e) :amount)))
+        0)))
+  12)
+(persist-test
+  "upcast works on the durable backend"
+  (let
+    ((db (persist/mock-durable (persist/mem-backend)))
+      (reg
+        (persist/register-upcaster (persist/upcasters) "placed" up-placed)))
+    (begin
+      (persist/append db "orders" "placed" 0 {:total 42})
+      (get
+        (persist/event-data
+          (nth (persist/read-upcast db "orders" reg) 0))
+        :amount)))
+  42)
diff --git a/lib/persist/upcast.sx b/lib/persist/upcast.sx
new file mode 100644
index 00000000..74bc0197
--- /dev/null
+++ b/lib/persist/upcast.sx
@@ -0,0 +1,44 @@
+; persist/upcast — event schema evolution. An append-only log keeps events
+; forever, so old events have old shapes. Rather than migrate stored data (you
+; can't rewrite history) or branch every projection on version, register an
+; upcaster per event type: a pure (event -> event) that lifts an old event to
+; the current shape. Reads pass through the registry so projections see ONE
+; shape. The registry is an immutable dict the consumer threads (no global
+; mutable state). Requires: lib/persist/event.sx, lib/persist/log.sx.
+
+(define persist/upcasters (fn () {}))
+(define persist/register-upcaster (fn (reg type fn) (assoc reg type fn)))
+
+; apply the registered upcaster for an event's type, or pass it through unchanged
+(define
+  persist/upcast
+  (fn
+    (reg e)
+    (let ((f (get reg (persist/event-type e)))) (if f (f e) e))))
+
+; read a stream with every event lifted to current shape
+(define
+  persist/read-upcast
+  (fn
+    (b stream reg)
+    (map (fn (e) (persist/upcast reg e)) (persist/read b stream))))
+
+; project over upcasted events — projections never see a legacy shape
+(define
+  persist/project-upcast
+  (fn
+    (b stream reg step seed)
+    (reduce step seed (persist/read-upcast b stream reg))))
+
+; helper: upcast an event's :data by merging in/overriding fields, keeping the
+; record's stream/seq/type/at. Common upcaster body.
+(define
+  persist/upcast-data
+  (fn
+    (e new-data)
+    (persist/event
+      (persist/event-stream e)
+      (persist/event-seq e)
+      (persist/event-type e)
+      (persist/event-at e)
+      (merge (persist/event-data e) new-data))))
diff --git a/plans/persist-on-sx.md b/plans/persist-on-sx.md
index a8f94222..15d8087a 100644
--- a/plans/persist-on-sx.md
+++ b/plans/persist-on-sx.md
@@ -42,7 +42,7 @@ read models (feeds, indices, audit logs) update incrementally.
 
 ## Status (rolling)
 
-`bash lib/persist/conformance.sh` → **162/162** (Phases 1–4 complete + extensions)
+`bash lib/persist/conformance.sh` → **171/171** (Phases 1–4 complete + extensions)
 
 ## Ground rules
 
@@ -167,11 +167,22 @@ over an in-process disk (the mock-IO harness).
   (all-or-nothing guarded by optimistic concurrency). For an order + its line
   items as one commit.
 
+- [x] `upcast.sx` — event schema evolution: register a pure `(event -> event)`
+  upcaster per type; `read-upcast`/`project-upcast` lift old events to the
+  current shape on read so projections see one shape. Immutable registry;
+  `upcast-data` helper merges new `:data` fields. Addresses the schema-evolution
+  trap without rewriting history.
+
 ## Consumers (post-foundation, not in scope here)
 feed/-log, flow store, mod/audit, search index, acl grants, identity sessions all
 become `persist` log or kv. Track each migration in that subsystem's plan.
 
 ## Progress log
+- **Ext: event schema evolution (171/171).** `upcast.sx` — per-type pure
+  `(event -> event)` upcasters in an immutable registry; `read-upcast`/
+  `project-upcast` lift legacy events to the current shape on read so
+  projections never branch on version. `upcast-data` merges new `:data` fields
+  keeping stream/seq/type/at. 9 tests incl. mixed old/new + durable.
 - **Ext: atomic batch append (162/162).** `batch.sx` — `persist/append-batch`
   commits `(type at data)` specs as one contiguous block (real cons-list, in
   order); `persist/append-batch-expect` checks the stream is still at expected