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Since .NET 4 is backward compatible, can't all app pools be set to use .NET 4 without changing your web site? Or will web.config's need to be updated simply from the app pool change?

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There are certain cases where you will need to update your web.config, in particular when using .NET 3.5 you will need to clear the web.config from all the section definitions that were added to it, for things like System.Web.Extensions or otherwise you will run into errors in the configuration telling you duplicate items in the collection.

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