I had a similar decision to make with the VB.net project. The solution was a compromise, I decided to work with dual VB and C #.
Updating VB.NET from 2005 to 2008 is an easy bit.
Added CSharp and VB folders in App_Code and
<codeSubDirectories> <add directoryName="CSharp" /> <add directoryName="VB" /> </codeSubDirectories>
to the compilation section of web.config
According to Kev, this is not as straightforward as you might expect, and you will probably encounter unexpected problems that make working with bilingual support the best solution.
I know this does not directly answer the question, but this is an alternative approach
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