>Q: In short, what would be the best way to design the original ASP app to make it easier to later rewrite (and very possibly) replace it with ASP .Net?

aspcompat=true

"Converting ASP to ASP.NET"
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dndotnet/html/convertasptoaspnet.asp

Personally, I think that rewriting the pages from scratch in .NET will be a better learning experience and yield better results.