18 Jul 2012
16 Feb 2012
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Do you need to preserve the format of the urls, or simply ensure that they are not lost?
Sitefinity allows you to specify alternate urls to each individual page and content item, which can certainly match your existing pages when they replace them.
Alternatively, you can use rewrite rules, which is especially helpful (and recommended) if your old urls use extensions such as .php or .aspx because Sitefinity uses no extensions in the url for better SEO.
A combination of the two (rewriting urls to remove the extensions then keeping the old urls as alternates) should probably allow you to upgrade your site without losing any existing SEO you've accumulated.
You can also convert your site in sections; the file-based html files, images, and folders can serve from the file system simultaneously along side the dynamic Sitefinity CMS pages.
I hope this is helpful!