Forcing StreamWriter to change Encoding

Just wrap it in a FileStream.

StreamWriter sw = new StreamWriter(
    new FileStream(saveFileDialog1.FileName, FileMode.Open, FileAccess.ReadWrite),
    Encoding.UTF8
);

If you want to append, use FileMode.Append instead.

You should also call Dispose() on a try/finally block, or use a using block to dispose the object when it exceeds the using scope:

using(
    var sw = new StreamWriter(
        new FileStream(saveFileDialog1.FileName, FileMode.Open, FileAccess.ReadWrite),
        Encoding.UTF8
    )
)
{
    sw.Write(sb.ToString());
}

This will properly close and dispose the streams across all exception paths.

UPDATE:

As per JinThakur’s comment below, there is a constructor overload for StreamWriter that lets you do this directly:

var sw = new StreamWriter(saveFileDialog1.FileName, false, Encoding.UTF8);

The second parameter specifies whether the StreamWriter should append to the file if it exists, rather than truncating it.