diff --git a/lib/model/migrations/20200428-01-allow-string-downcast.js b/lib/model/migrations/20200428-01-allow-string-downcast.js
new file mode 100644
index 000000000..4e9955f4e
--- /dev/null
+++ b/lib/model/migrations/20200428-01-allow-string-downcast.js
@@ -0,0 +1,70 @@
+// Copyright 2020 ODK Central Developers
+// See the NOTICE file at the top-level directory of this distribution and at
+// https://github.com/opendatakit/central-backend/blob/master/NOTICE.
+// This file is part of ODK Central. It is subject to the license terms in
+// the LICENSE file found in the top-level directory of this distribution and at
+// https://www.apache.org/licenses/LICENSE-2.0. No part of ODK Central,
+// including this file, may be copied, modified, propagated, or distributed
+// except according to the terms contained in the LICENSE file.
+
+
+const up = async (db) => {
+  await db.raw(`
+create or replace function check_field_collisions() returns trigger as $check_field_collisions$
+  declare extant int;
+  declare extformid int;
+  declare extpath text;
+  declare exttype text;
+  begin
+    -- factoring the repeated joins here into a CTE explodes the cost by 10x
+
+    select count(distinct type), form_fields."formId", form_fields.path into extant, extformid, extpath
+      from form_fields
+
+      -- finds canonical formDefIds (published, or active draft)
+      left outer join (select id from form_defs where "publishedAt" is not null) as form_defs
+        on form_defs.id = form_fields."formDefId"
+      left outer join (select id, "draftDefId" from forms) as forms
+        on forms."draftDefId" = form_fields."formDefId"
+
+      -- weeds out paths whose latest def indicates they are a string. first figure
+      -- out latest def, then knock out latest strings from conflict detection.
+      inner join
+        (select form_fields."formId", max("formDefId") as "latestDefId" from form_fields
+          -- this is a repeat of the above canonical-def subquery
+          left outer join (select id from form_defs where "publishedAt" is not null) as ifds
+            on ifds.id = form_fields."formDefId"
+          left outer join (select id, "draftDefId" from forms) as ifs
+            on ifs."draftDefId" = form_fields."formDefId"
+          where ifs.id is not null or ifds.id is not null
+          group by form_fields."formId"
+        ) as tail
+        on tail."formId" = form_fields."formId"
+      inner join
+        (select "formDefId", path from form_fields where type != 'string') as nonstring
+        on "latestDefId" = nonstring."formDefId" and form_fields.path = nonstring.path
+
+      where forms.id is not null or form_defs.id is not null
+      group by form_fields."formId", form_fields.path having count(distinct type) > 1;
+
+    if extant > 0 then
+      select type into exttype
+        from form_fields
+        where "formId" = extformid and path = extpath
+        order by "formDefId" desc
+        limit 1
+        offset 1;
+
+      raise exception using message = format('ODK05:%s:%s', extpath, exttype);
+    end if;
+
+    return NEW;
+  end;
+$check_field_collisions$ language plpgsql;
+`);
+};
+
+const down = () => {}; // no. would cause problems.
+
+module.exports = { up, down };
+
diff --git a/test/integration/api/forms.js b/test/integration/api/forms.js
index 9cf789edd..79b536181 100644
--- a/test/integration/api/forms.js
+++ b/test/integration/api/forms.js
@@ -1386,7 +1386,7 @@ describe('api: /projects/:id/forms', () => {
       it('should complain about field conflicts', testService((service) =>
         service.login('alice', (asAlice) =>
           asAlice.post('/v1/projects/1/forms/simple/draft')
-            .send(testData.forms.simple.replace('type="int"', 'type="string"'))
+            .send(testData.forms.simple.replace('type="int"', 'type="date"'))
             .set('Content-Type', 'application/xml')
             .expect(400)
             .then(({ body }) => {
@@ -1405,7 +1405,7 @@ describe('api: /projects/:id/forms', () => {
             .then(() => asAlice.post('/v1/projects/1/forms/simple/draft/publish')
               .expect(200))
             .then(() => asAlice.post('/v1/projects/1/forms/simple/draft')
-              .send(testData.forms.simple.replace('type="int"', 'type="string"'))
+              .send(testData.forms.simple.replace('type="int"', 'type="date"'))
               .set('Content-Type', 'application/xml')
               .expect(400)
               .then(({ body }) => {
@@ -1413,6 +1413,52 @@ describe('api: /projects/:id/forms', () => {
                 body.details.should.eql({ path: '/age', type: 'int' });
               })))));
 
+      it('should not complain on downcast to string', testService((service) =>
+        service.login('alice', (asAlice) =>
+          asAlice.post('/v1/projects/1/forms/simple/draft')
+            .send(testData.forms.simple.replace('type="int"', 'type="string"'))
+            .set('Content-Type', 'application/xml')
+            .expect(200))));
+
+      it('should complain on upcast from string', testService((service) =>
+        service.login('alice', (asAlice) =>
+          asAlice.post('/v1/projects/1/forms/simple/draft')
+            .send(testData.forms.simple.replace('name" type="string"', 'name" type="int"'))
+            .set('Content-Type', 'application/xml')
+            .expect(400)
+            .then(({ body }) => {
+              body.code.should.equal(400.17);
+              body.details.should.eql({ path: '/name', type: 'string' });
+            }))));
+
+      it('should not complain on double-downcast to string', testService((service) =>
+        service.login('alice', (asAlice) =>
+          asAlice.post('/v1/projects/1/forms/simple/draft')
+            .send(testData.forms.simple.replace('type="int"', 'type="string"'))
+            .set('Content-Type', 'application/xml')
+            .expect(200)
+            .then(() => asAlice.post('/v1/projects/1/forms/simple/draft/publish?version=two')
+              .expect(200))
+            .then(() => asAlice.post('/v1/projects/1/forms/simple/draft')
+              .expect(200)))));
+
+      it('should complain on upcast once downcasted', testService((service) =>
+        service.login('alice', (asAlice) =>
+          asAlice.post('/v1/projects/1/forms/simple/draft')
+            .send(testData.forms.simple.replace('type="int"', 'type="string"'))
+            .set('Content-Type', 'application/xml')
+            .expect(200)
+            .then(() => asAlice.post('/v1/projects/1/forms/simple/draft/publish?version=two')
+              .expect(200))
+            .then(() => asAlice.post('/v1/projects/1/forms/simple/draft')
+            .send(testData.forms.simple)
+              .set('Content-Type', 'application/xml')
+              .expect(400)
+              .then(({ body }) => {
+                body.code.should.equal(400.17);
+                body.details.should.eql({ path: '/age', type: 'string' });
+              })))));
+
       it('should not complain about discarded draft field conflicts', testService((service) =>
         service.login('alice', (asAlice) =>
           asAlice.post('/v1/projects/1/forms/simple/draft')