As has already been pointed out, not all objects that are binary serializable, are also XML serializable (for example, anything with TimeSpan).
But if you can deal with that pretty serious flaw, then the approach I would probably take is to create my own interface. I would then have 2 classes that implement it, one wrapping the binary formatter and one wrapping the XML serializer. To make life easier, have the interface very similar to the binary formatter in terms of method names your app uses and parameters so that you can replace occurrences of the concrete binary formatter relatively painlessly.