There are a few challenges you may face, the first is building the GUI so that it has low coupling, if the GUI is built so when you change the platform you have to completely redesign it you will need to publish it as a separate app. This can be overcome as you design a link which says the platform is about to be decommissioned click here to be taken to the new version on the app store (but this will result in your use number dropping by circa 25%).
You have already touched on the need to retain user data, and clearly exporting the back end database to CSV or similar would allow you to import user data in the app, just remember when it comes to building the final platform you need to consider a way of importing legacy users into the back end.
In summary providing you build the app so you can still call the same SOAP method and own the URL, migrating to another platform is simple, as you just point the URL to your new back end, and ensure when you build the new platform you use the same SOAP methods.
Please let me know if you need anything clarified
Its the mechanism \ protocol behind the web service, the webpage here explains it rather well.
I havent heard back from you in the last few day, appreciate if you can rate the service received, and let me know if you have any further questions.