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So if you change the url, it is a new url and it goes through the regular hazing process that Google imposes on new things. Where you must change a url for already ranking content, make sure that "old" urls no longer resolve with 200 OK. A 301 permanent redirect or a 404 is fine.
The standard procedure is to *internally rewrite* your old static URLs to the form needed by your new (dynamic) page-generation script. This can be done using ISAPI Rewrite on IIS, or with mod_rewrite on Apache [webmasterworld.com] server.
There is no need to change URLs in this case. As a matter of fact, with a little site-structure planning, there is never any need to change a URL. Comment from Tim Berners-Lee, the inventor of the Web [w3.org]
The second you have to do however, is making sure that Google gets no access directly to the new .aspx URL. For example:
Your old URL is www.example.com/file.htm
Your new URL is www.example.com/dynamic/file.aspx
When Google manages to spider the /dynamic/file.aspx URL, it will compare it with the file.htm version it already has in its index, and a duplicate content penalty might be the result. I am suffering on one site myself from this phenomem. On this site, Google has found my wanted www.example.com/file.html versions, but for some strange reason also managed to access the site with www.example.com/index.php?... URLs. Almost the complete site went supplemental because of this. I still don't know how Google managed to find the /index.php?... series of URLs, but it is almost impossible for me to remove them from the index, once Google knows they are there.
I don't mean to hijack this thread but I'm having the same problem with Yahoo and I'm guessing soon it will be a Google problem also. All my URL's are static .html using Apache and mod-rewrite but somehow Yahoo is managing to index www.mysite/php?=...This has got to be a major problem. I'm thinking there must be a way to solve this with .htaccess but I haven't quite got it yet. Anyway, it's something to be aware of when changing your site structure.
This might cause problems on sites with high traffic, but for me it works perfectly fine. Is there any reason, why a similar construction would not work with asp?