If you replace an old site with a new one while keeping the existing domain name, you will want to create 301 redirects for all of the pages which no longer exist in the name of not losing your search engine rankings.

The way to do this in C# (for a given page or route) is given here. At Headspring, I moved a site from .jsp (JavaServer Pages) to .aspx (ASP.NET web forms) and I recall that I had to jump through some hoops to get fake .jsp pages (which only contained 301 redirects) to run as .aspx pages in an ASP.NET environment. I recreated every page I threw away from the old site as a dummy page with a redirect at the new site. I believe I had to "OK" the .jsp extension somehow in the Web.config file. We used a third party for the hosting so I did not have to alter what .jsp did in IIS myself, but I wonder if I put in a request for as much with the third party. That would make sense. I can't recall.