18 Jul 2012
11 Mar 2009
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we just did this with much success! it was actually pretty simple.
the basic process for me was to open the website in visual studio. If you haven't done this before, it will actually prompt you to upgrade the project to 3.5 (modifying the web.config and assemblies accordingly), if you click yes it will convert your project to asp.net 3.5. Alternatively if you already opened your site in visual studio, you can right click the project and under properties->build change the target framework to 3.5 and it will do the same thing...
next you'll want to replace the site with the .net 3.5 version of sitefinity (make sure you have a backup so you can bring back your customizations if any). I also use notepad++ compare tool to compare the distributed web.config with my own and make the appropriate changes.
you might need to change some assembly references in your code if you're using something like the ajax extensions, but for me it really was this simple. remember that asp.net 3.5 is actually the same runtime as asp.net 2.0 so the changes to upgrade aren't all that crazy.
i hope this was helpful!