You want to deliver one update that updates all versions to the latest? Have I got that correct?
If that's what you mean, then you can do it rather simply. Just create a single version in wyBuild. Add your MSI to the temp files section, and set it to execute either before or after the update (it doesn't matter in your case):
[attachment=1]version.png[/attachment]
With this single version you'll be able to update all existing versions.
Then when you have a new version to release, simply change the version number in the text box to the latest version:
[attachment=0]new-version.png[/attachment]
Tell me if this helps.