Dave
To the best of my knowledge, ReportNet does not use any of the component of it's predecessor app. It has a new client piece like the Architect called the Framework Manager that does the metadata repository work and ER kind of modeling.
Apparently, one cannot migrate from any of the earlier versions of Cognos BI to ReportNet. One has to begin from scratch by re-designing the metadata architecture and all the reports therefrom.
From what I have heard, it is basically Impromptu, IWR, Architect and Cognos Query all rolled in to one app, and much more and you can run queries off multiple data sources.