I'm having a related problem. I was using the default Midnight skin and when I updated the dialog to remove the modify button everything seems to work well except in Japanese. In that one language, on that one pane it uses the default skin instead ...
I have this same problem. I work around it by checking the registry to see if my product is already on the system. If it is I take the upgrade path. If it isn't I take the install path even though the property indicates it should be an upgrade.