Here are a few things to consider: different fuel system, different electrical system, different cowl to meet cooling requirements, higher empty weight/reduced useful load with iS engine, not to mention the expense of buying a new engine and not seeing a large increase in performance compared to the ULS engine that’s currently installed.
That’s not to say the iS doesn’t have advantages over the ULS; it does. But it doesn’t seem to make much sense from an economic standpoint if you have a perfectly good ULS already installed.