I started building an app with protobuf 2.2.0 as it was the latest. Now I'm considering upgrading to protobuf 2.4.0a which is the latest. If I do so will the messages generated by one version of the app still be readable by the other version for the same schema? Or would I be breaking something?
That should be fine. Version 2.1.0 introduced packed repeated fields, but I don't believe there have been any wire format changes since then.