In my master thesis i aimed at improving an exisitng system UI. the solution i proposed is analysis and evaluate the system based on the documentation and running web site and next develop scenarios that address shortcommings. the scenarios help to derive use cases and task flow and navigation to come up with wireframes. note that it doesn't neccesary for me to develop data model.
"Official" UCD is described in ISO 9241-210:2010 http://www.iso.org/iso/catalogue_detail.htm?csnumber=52075
I've never heard of anyone actually using it, but if you are looking for formal UX Design methods then that's probably the place to start.