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+### Breaking
+
+- Change internal representation of `OneEraGenTxId xs` from `NS WrapGenTxId xs` to `ShortByteString`. This breaks backwards compatibility for serialisation of Byron-era `GenTxId`s, and introduces new hard-fork-specific node-to-client and node-to-node versions, `HardForkSpecificNodeToClientVersion4` and `HardForkSpecificNodeToNodeVersion2`. Currently-running networks which require the ability to submit Byron transactions should ensure that all nodes are upgraded simultaneously, or these nodes may be unable to communicate with one another.
+- Change internal representation of Byron-era `GenTxId`s from a sum of the different possible Byron IDs (transaction ID, certification delegation ID, etc.) to `Hash`. In order to determine the type of a Byron `GenTxId`, you'll now need to find the transaction that the ID refers to and check its type.