I think you might be mixing up ChangeSets with RevisionHistory. If you're looking to track Revisions on a Rally Artifact, you should look at the RevisionHistory collection. ChangeSets are used by Rally's source code connectors to associate source code commits to various artifacts in Rally.
You might find this answer:
Instructive for dealing with RevisionHistory in SOAP. The example is for User Stories, but is equally applicable to Test Cases.
Unfortunately, you can't query on RevisionHistory, so you would need to retrieve all the Test Cases that could potentially have matches against your search, and do the filtering for the TestCase creator in your SOAP client. You would need to check the Author attribute for Revision 0:
0 Original revision
2013-Feb-18 09:23:11 AM America/Denver Sam Tester
And verify that it matches your criteria.
Just a heads-up, especially if you're just getting started with building your integration, I'd highly recommend using one of Rally's .NET REST SDK instead of SOAP.
REST is more robust, more performant, and, Webservices API 1.43 was the final API release to have SOAP support, and 1.43 has about 9 months to go (as of 9/2013) before deprecation. Webservices 2.x is REST-only, so using REST will be essential to anyone wanting new Webservices features moving forward.