Hi,
It's a little too generic to specify "Cognos 7". Unlike Cognos 8/10, the Cognos 7 release is a suite of loosely coupled products, each with a different architecture. You have Impromptu Client, PowerPlay Client, Impromptu Web Reports, PowerPlay Enterprise Server, NoticeCast, Cognos Query, Visualizer client and web, Upfront portal etc. There are different strategies for migrating different parts of the C7 suite (or not migrating, as the case may be).
Regards,
MF.