diff --git a/src/Postgres/README.md b/src/Postgres/README.md
index 8e6ba1c..ff442c9 100644
--- a/src/Postgres/README.md
+++ b/src/Postgres/README.md
@@ -11,6 +11,10 @@ This package provides a lightweight document library backed by [PostgreSQL](http
 - Use `Task`-based async for all data access functions
 - Use building blocks for more complex queries
 
+## Upgrading from v3
+
+There is a breaking API change for `ByField` (C#) / `byField` (F#), along with a compatibility namespace that can mitigate the impact of these changes. See [the migration guide](https://bitbadger.solutions/open-source/relational-documents/upgrade-from-v3-to-v4.html) for full details.
+
 ## Getting Started
 
 Once the package is installed, the library needs a data source. Construct an `NpgsqlDataSource` instance, and provide it to the library:
diff --git a/src/Sqlite/README.md b/src/Sqlite/README.md
index 7f0651e..fdd8a46 100644
--- a/src/Sqlite/README.md
+++ b/src/Sqlite/README.md
@@ -11,6 +11,10 @@ This package provides a lightweight document library backed by [SQLite](https://
 - Use `Task`-based async for all data access functions
 - Use building blocks for more complex queries
 
+## Upgrading from v3
+
+There is a breaking API change for `ByField` (C#) / `byField` (F#), along with a compatibility namespace that can mitigate the impact of these changes. See [the migration guide](https://bitbadger.solutions/open-source/relational-documents/upgrade-from-v3-to-v4.html) for full details.
+
 ## Getting Started
 
 Once the package is installed, the library needs a connection string. Once it has been obtained / constructed, provide it to the library: