I was just asked how to install Commerce Server 2002 on top of SQL Server 2008 – and fix the things that broke. Unfortunately, you just can’t do that. Or support .NET Framework newer than 1.1. Or run on an operating system newer than Windows Server 2003. Or run in 64-bit
Why? A lot changed between the time Commerce Server 2002 shipped and in the latest generation of platform changes. Many of these changes were disruptive – and required extensive modification in Commerce Server 2007. The changes are substantial enough – that you may as well upgrade, as the cost of back-porting the newer platform support would be prohibitive.