How can agents support messages for other (potentially not yet created) versions of SIF?

Backwards compatibility standards within a major version of SIF are very strict. Getting messages of a different version back and consuming their data objects should be trivial. Depending on your implementation, it can be zero extra work. For instance, if you parse the message and use several specific XPaths to pull out the data your application commits to its data base, your 2.0r1 agent should be able to consume any SIF 2 data object it receives. Additionally, when your agent publishes a change, follow the best practice of omitting all unchanged elements and you will not misinform your peers.