add a test for downgrading askrene datastore

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index c93221c..eec91fb 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -465,6 +465,7 @@ jobs:
           LIGHTNINGD_POSTGRES_NO_VACUUM: 1
           VALGRIND: ${{ matrix.VALGRIND }}
           PREV_LIGHTNINGD: old-cln/usr/bin/lightningd
+          CLN_PREV_VERSION: v26.06
           PYTEST_OPTS: ${{ env.PYTEST_OPTS_BASE }}
         run: |
           env
diff --git a/tests/test_downgrade.py b/tests/test_downgrade.py
index f48686b..c791366 100644
--- a/tests/test_downgrade.py
+++ b/tests/test_downgrade.py
@@ -1,10 +1,14 @@
 from fixtures import *  # noqa: F401,F403
 from utils import (
     TIMEOUT,  # noqa: F401
-    first_scid, only_one,
+    first_scid,
+    only_one,
+    generate_gossip_store,
+    GenChannel,
 )
 
 import os
+import pytest
 import subprocess
 
 # From the binary:
@@ -13,6 +17,22 @@ ERROR_DBFAIL = 2
 ERROR_USAGE = 3
 # ERROR_INTERNAL = 99
 
+PREV_LIGHTNINGD = os.getenv('PREV_LIGHTNINGD')
+CLN_PREV_VERSION = os.getenv('CLN_PREV_VERSION')
+
+
+def direction(src, dst):
+    """BOLT 7 direction: 0 means from lesser encoded id"""
+    if src < dst:
+        return 0
+    return 1
+
+
+def scid_dir(nodemap, node1_idx, node2_idx, chan_idx):
+    """Get short_channel_id_dir for a channel in generate_gossip_store format"""
+    dir_val = direction(nodemap[node1_idx], nodemap[node2_idx])
+    return f"{node1_idx}x{node2_idx}x{chan_idx}/{dir_val}"
+
 
 def downgrade_cmdline(node):
     # lightning-downgrade understands a subset of the options
@@ -111,3 +131,136 @@ def test_downgrade_bias(node_factory, executor):
                          stderr=subprocess.PIPE)
     _, err = p.communicate(timeout=TIMEOUT)
     assert p.returncode == 0
+
+
+@pytest.mark.skipif(not CLN_PREV_VERSION, reason="Depends on CLN_PREV_VERSION")
+@pytest.mark.skipif(not PREV_LIGHTNINGD, reason="Depends on PREV_LIGHTNINGD")
+def test_downgrade_impressions(node_factory, executor):
+    """Impressions are added since v26.09, the downgrade tool should remove
+    them while the rest of the askrene data remains."""
+    cap_msat = 1000_000_000
+    gsfile, nodemap = generate_gossip_store(
+        [GenChannel(0, 1, capacity_sats=cap_msat // 1000)]
+    )
+    l1 = node_factory.get_node(
+        gossip_store_file=gsfile.name, options={"database-upgrade": True}
+    )
+
+    chan_dir = scid_dir(nodemap, 0, 1, 0)
+    l1.rpc.askrene_create_layer(layer="test_downgrade", persistent=True)
+    expect = {
+        "biases": [],
+        "channel_updates": [],
+        "constraints": [],
+        "created_channels": [],
+        "disabled_nodes": [],
+        "impressions": [],
+        "layer": "test_downgrade",
+        "node_biases": [],
+        "persistent": True,
+    }
+    ret = l1.rpc.askrene_inform_channel(
+        "test_downgrade", chan_dir, 1000000, "succeeded"
+    )
+
+    expect["impressions"] = [
+        {
+            "amount_msat": 1000000,
+            "short_channel_id_dir": chan_dir,
+            "timestamp": ret["impressions"][0]["timestamp"],
+        }
+    ]
+
+    ret = l1.rpc.askrene_inform_channel(
+        "test_downgrade", chan_dir, 500000, "constrained"
+    )
+    expect["constraints"] = [
+        {
+            "maximum_msat": 499999,
+            "short_channel_id_dir": chan_dir,
+            "timestamp": ret["constraints"][0]["timestamp"],
+        }
+    ]
+    ret = l1.rpc.askrene_inform_channel(
+        "test_downgrade", chan_dir, 20000, "unconstrained"
+    )
+    expect["constraints"].append(
+        {
+            "minimum_msat": 20000,
+            "short_channel_id_dir": chan_dir,
+            "timestamp": ret["constraints"][0]["timestamp"],
+        }
+    )
+
+    A_NODE = "020000000000000000000000000000000000000000000000000000000000000001"
+    ret = l1.rpc.askrene_create_channel(
+        "test_downgrade", A_NODE, l1.info["id"], "16000000x1x1", "100000000sat"
+    )
+    expect["created_channels"] = [
+        {
+            "source": A_NODE,
+            "destination": l1.info["id"],
+            "short_channel_id": "16000000x1x1",
+            "capacity_msat": 100000000000,
+        }
+    ]
+
+    l1.rpc.askrene_update_channel(
+        layer="test_downgrade",
+        short_channel_id_dir="16000000x1x1/0",
+        enabled=True,
+        htlc_minimum_msat=1000,
+    )
+    expect["channel_updates"] = [
+        {
+            "enabled": True,
+            "htlc_minimum_msat": 1000,
+            "short_channel_id_dir": "16000000x1x1/0",
+        }
+    ]
+    ret = l1.rpc.askrene_bias_channel(
+        layer="test_downgrade", short_channel_id_dir="16000000x1x1/0", bias=3
+    )
+    expect["biases"] = [
+        {
+            "timestamp": ret["biases"][0]["timestamp"],
+            "bias": 3,
+            "short_channel_id_dir": "16000000x1x1/0",
+        }
+    ]
+    B_NODE = "020000000000000000000000000000000000000000000000000000000000000002"
+
+    l1.rpc.askrene_disable_node(layer="test_downgrade", node=B_NODE)
+    expect["disabled_nodes"] = [B_NODE]
+
+    ret = l1.rpc.askrene_bias_node(
+        layer="test_downgrade", direction="in", node=A_NODE, bias=-2
+    )
+    expect["node_biases"] = [
+        {
+            "in_bias": -2,
+            "out_bias": 0,
+            "node": A_NODE,
+            "timestamp": ret["node_biases"][0]["timestamp"],
+        }
+    ]
+    assert l1.rpc.askrene_listlayers("test_downgrade") == {"layers": [expect]}
+
+    cmd_line = downgrade_cmdline(l1)
+
+    l1.stop()
+
+    p = subprocess.run(cmd_line, timeout=TIMEOUT, capture_output=True, text=True)
+    assert p.returncode == 0
+    if CLN_PREV_VERSION == "v26.06":
+        assert "Downgrade to v26.06 succeeded.  Committing." in p.stdout
+
+    # we need to disable the schema checks, this node uses an old API
+    l1.rpc.jsonschemas = {}
+    # in the old node the impressions field doesn't even exist
+    if CLN_PREV_VERSION == "v26.06":
+        del expect["impressions"]
+    l1.daemon.executable = PREV_LIGHTNINGD
+
+    l1.start()
+    assert l1.rpc.askrene_listlayers("test_downgrade") == {"layers": [expect]}
diff --git a/wallet/migrations.c b/wallet/migrations.c
index 16b663a..2ee8c3b 100644
--- a/wallet/migrations.c
+++ b/wallet/migrations.c
@@ -1182,6 +1182,7 @@ static const struct db_migration dbmigrations[] = {
      * writes stop in the release that removes chaintopology, freezing all
      * the legacy tables at the same height. */
     {NULL, migrate_backfill_bwatch_tables, NULL, NULL},
+    /* ^v26.09 */
 };
 
 const struct db_migration *get_db_migrations(size_t *num)