diff --git a/isic/ingest/migrations/0043_alter_rcmcase_id.py b/isic/ingest/migrations/0043_alter_rcmcase_id.py
new file mode 100644
index 00000000..73fb5f22
--- /dev/null
+++ b/isic/ingest/migrations/0043_alter_rcmcase_id.py
@@ -0,0 +1,51 @@
+# Generated by Django 5.2.3 on 2026-06-09 21:07
+
+from django.db import migrations, models
+from django.db.models import F, Value
+from django.db.models.functions import Concat
+
+import isic.ingest.models.rcm_case
+
+
+def add_prefix(apps, schema_editor):
+    Accession = apps.get_model("ingest", "Accession")
+    RcmCase = apps.get_model("ingest", "RcmCase")
+    MetadataVersion = apps.get_model("ingest", "MetadataVersion")
+
+    Accession.objects.filter(rcm_case__isnull=False).exclude(
+        rcm_case__id__startswith="IRCM_"
+    ).update(rcm_case_id=Concat(Value("IRCM_"), F("rcm_case_id")))
+
+    RcmCase.objects.exclude(id__startswith="IRCM_").update(id=Concat(Value("IRCM_"), F("id")))
+
+    # MetadataVersion snapshots store the public (generated) id under
+    # rcm_case->>'external'. The 'internal' key holds the internal id and is
+    # unchanged.
+    metadata_versions = MetadataVersion.objects.filter(rcm_case__has_key="external").exclude(
+        rcm_case__external__startswith="IRCM_"
+    )
+    to_update = []
+    for metadata_version in metadata_versions.iterator():
+        metadata_version.rcm_case["external"] = f"IRCM_{metadata_version.rcm_case['external']}"
+        to_update.append(metadata_version)
+    MetadataVersion.objects.bulk_update(to_update, ["rcm_case"], batch_size=1000)
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("ingest", "0042_blob_verbose_names"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="rcmcase",
+            name="id",
+            field=models.CharField(
+                default=isic.ingest.models.rcm_case._default_id,  # noqa: SLF001
+                max_length=12,
+                primary_key=True,
+                serialize=False,
+            ),
+        ),
+        migrations.RunPython(add_prefix, migrations.RunPython.noop, elidable=True),
+    ]
diff --git a/isic/ingest/models/rcm_case.py b/isic/ingest/models/rcm_case.py
index 481a4858..d3327f24 100644
--- a/isic/ingest/models/rcm_case.py
+++ b/isic/ingest/models/rcm_case.py
@@ -6,7 +6,7 @@
 
 def _default_id():
     while True:
-        rcm_case_id = f"{secrets.randbelow(9999999):07}"
+        rcm_case_id = f"IRCM_{secrets.randbelow(9999999):07}"
         # This has a race condition, so the actual creation should be retried or wrapped
         # in a select for update on the rcm_case table
         if not RcmCase.objects.filter(id=rcm_case_id).exists():
@@ -17,7 +17,7 @@ class RcmCase(models.Model):
     id = models.CharField(
         primary_key=True,
         default=_default_id,
-        max_length=7,
+        max_length=12,
     )
     cohort = models.ForeignKey("Cohort", on_delete=models.CASCADE, related_name="rcm_cases")
     private_rcm_case_id = models.CharField(max_length=255)