So if someone ever needs to change an session ID for some other reason they can implement SessionIdChangeEvent separate from the events we are creating here. Okay. Final proposal. I think you'll be okay with it:

    SessionFixationProtectionEvent
        String getOldSessionId()
        String getNewSessionId()
    SessionFixationProtectionMigrationEvent extends SessionFixationProtectionEvent
        boolean sessionAttributesWereMigrated()

Notice I changed `SessionMigrationProtectionEvent` to `SessionFixationProtectionMigrationEvent`. `SessionMigrationProtectionEvent` sounds like we're protected against session migrations. Awkward phrasing.