hmm.. let me explain.
It is not possible to change a filename (nor the extension) of a file with .NET. This just is not implemented.
BUT you can do as follows: Copy the content of the existing file into a new file (name it as you want), afterwards delete the source file.
private void RenameFile(string filename, string newFileName)
{
File.WriteAllText(newFileName, File.ReadAllText(filename, Encoding.Default));
File.Delete(filename);
}
regards Patrick