Try re-building wyUpdate for the old version of your app. This will generate a new "client.wyc" file associated with the old version of your app. (Make sure you select the old version from the drop down on the build page).
The version numbers are stored in the client.wyc file. So if you already tested an update to the latest version, the client.wyc is mow associated with the new version.
Just rebuild wyUpdate and copy the new wyUpdate.exe and client.wyc to the folder.