in most GUI software the "interface" (mapping between model state, user input and visual presentation) is the majority of the code. Changing the interface isn't a detail, it's everything. Tablets have (a) no keyboard (b) no concept of pointer motion without a drag (c) no concept of hover (d) much less screen real estate (e) multitouch (in some cases, at least). that is enough of a set of changes that any desktop application which took advantage of some of the scale of a desktop system will likely need to be reconceived fairly deeply.