Add 2.51->52 breaking incompatibility to CHANGES.md (#32753)

* Add 2.51->52 breaking incompatibility to CHANGES.md

* Add blog

CHANGES.md

@@ -485,6 +485,7 @@ classes finally moved to `extensions/avro`. In case if it's still required to us
 as a workaround, a copy of "old" `CountingSource` class should be placed into a project code and used directly
 ([#25252](https://github.com/apache/beam/issues/25252)).
 * Renamed `host` to `firestoreHost` in `FirestoreOptions` to avoid potential conflict of command line arguments (Java) ([#29201](https://github.com/apache/beam/pull/29201)).
+* Transforms which use `SnappyCoder` are update incompatible with previous versions of the same transform (Java) on some runners. This includes PubSubIO's read ([#28655](https://github.com/apache/beam/pull/28655#issuecomment-2407839769)).
 
 ## Bugfixes
 
@@ -502,6 +503,7 @@ as a workaround, a copy of "old" `CountingSource` class should be placed into a
 * MLTransform drops the identical elements in the output PCollection. For any duplicate elements, a single element will be emitted downstream. ([#29600](https://github.com/apache/beam/issues/29600)).
 * Some Python pipelines that run with 2.52.0-2.54.0 SDKs and use large materialized side inputs might be affected by a performance regression. To restore the prior behavior on these SDK versions, supply the `--max_cache_memory_usage_mb=0` pipeline option. (Python) ([#30360](https://github.com/apache/beam/issues/30360)).
 * Users who lauch Python pipelines in an environment without internet access and use the `--setup_file` pipeline option might experience an increase in pipeline submission time. This has been fixed in 2.56.0 ([#31070](https://github.com/apache/beam/pull/31070)).
+* Transforms which use `SnappyCoder` are update incompatible with previous versions of the same transform (Java) on some runners. This includes PubSubIO's read ([#28655](https://github.com/apache/beam/pull/28655#issuecomment-2407839769)).
 
 # [2.51.0] - 2023-10-03
 

website/www/site/content/en/blog/beam-2.52.0.md

@@ -52,6 +52,7 @@ classes finally moved to `extensions/avro`. In case if it's still required to us
 as a workaround, a copy of "old" `CountingSource` class should be placed into a project code and used directly
 ([#25252](https://github.com/apache/beam/issues/25252)).
 * Renamed `host` to `firestoreHost` in `FirestoreOptions` to avoid potential conflict of command line arguments (Java) ([#29201](https://github.com/apache/beam/pull/29201)).
+* Transforms which use `SnappyCoder` are update incompatible with previous versions of the same transform (Java) on some runners. This includes PubSubIO's read ([#28655](https://github.com/apache/beam/pull/28655#issuecomment-2407839769)).
 
 ## Bugfixes
 
@@ -64,6 +65,12 @@ as a workaround, a copy of "old" `CountingSource` class should be placed into a
 * Fixed [CVE-2023-39325](https://www.cve.org/CVERecord?id=CVE-2023-39325) (Java/Python/Go) ([#29118](https://github.com/apache/beam/issues/29118)).
 * Mitigated [CVE-2023-47248](https://nvd.nist.gov/vuln/detail/CVE-2023-47248)  (Python) [#29392](https://github.com/apache/beam/issues/29392).
 
+## Known issues
+* MLTransform drops the identical elements in the output PCollection. For any duplicate elements, a single element will be emitted downstream. ([#29600](https://github.com/apache/beam/issues/29600)).
+* Some Python pipelines that run with 2.52.0-2.54.0 SDKs and use large materialized side inputs might be affected by a performance regression. To restore the prior behavior on these SDK versions, supply the `--max_cache_memory_usage_mb=0` pipeline option. (Python) ([#30360](https://github.com/apache/beam/issues/30360)).
+* Users who lauch Python pipelines in an environment without internet access and use the `--setup_file` pipeline option might experience an increase in pipeline submission time. This has been fixed in 2.56.0 ([#31070](https://github.com/apache/beam/pull/31070)).
+* Transforms which use `SnappyCoder` are update incompatible with previous versions of the same transform (Java) on some runners. This includes PubSubIO's read ([#28655](https://github.com/apache/beam/pull/28655#issuecomment-2407839769)).
+
 ## List of Contributors
 
 According to git shortlog, the following people contributed to the 2.52.0 release. Thank you to all contributors!