Redirection from ASP to ASP.NET

I have a large website built in ASP but now want to change it to ASP.net .I want to know that it has 200 pages index in Google with .asp extension.What is most suitable way to move .asp pages to .aspx pages without effecting SEO.As few pages are ranked in Google with .asp extension.

few more questions

1-Google will de-rank if we redirect using 302 on each page header .asp to .aspx ?

2-302 redirection will automatically update google index .asp to .aspx ?

 

In this case google will treat your pages as a new pages because the URL will be New.

 

If your asp pages are on search page 3 or deeper, I wouldn't bother much. Just go ahead and create new aspx site, properly optimizing it as development goes, and replace the old site once the new one is ready.

If your asp pages are highly ranked by Google, you have a couple of choices:
a) create the new aspx version of the site. Keep the existing site running where it is, unchanged, but add a link to the new version of the site. After the new version gains popularity and rankings, switch users to the new version.
b) create the new aspx version the way so the aspx pages replicate the old asp design, and perhaps use the asp http handler.

In any way Google will likely figure out that you have the new site, via http headers or somehow else, and you might lose a bit in ranking, but aspx will eventually win, since it's faster and more versatile than asp.