It is difficult to answer the question without being able to look at the application, but here are some guidelines:
1) If the application uses proper XML syntax, which is not a requirement for imode, but is a good practice, then the conversion should be quite straightforward. If proper XML syntax was not used, there will be some pain.
2) If the application does not overly rely on proprietary imode tags, it will help a great deal.
3) The formatting will almost certainly be different as there is no support for CSS. Because XHTML supports CSS, the application will look better.