Apache Pegasus 2.1.0 is a feature release. The change-list is summarized here: https://github.com/apache/incubator-pegasus/issues/577.
2.1.0 is not compatible in storage format with previous versions. ReplicaServers upgraded to this version can't rollback to any previous releases.
2.1.0 can only be upgraded from 2.0.0. ReplicaServers with version 1.x should upgrade to 2.0.0 firstly before upgrading to 2.1.0.