Worst case: you (re)test on every platform / software configuration.
You may be able to reduce the effort with some kind of (documented) compatibility / risk analysis. Have you looked at the latest FDA guidance on the subject (http://www.fda.gov/MedicalDevices/De...cm263280.htm)?
You MAY be able to limit your liability by limiting the platforms / configurations on which the app is verified. That may be a marketing limitation but if people's lives / health are at risk, it may be a smart move.
It's a VERY good and timely question as these platforms get smarter and are used in more situations like this. These are just my thoughts; hopefully others will chime in with their thoughts.