MSWord Find & Replace method changes formatting

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C#3.5

MS-Word

foreach (Microsoft.Office.Interop.Word.Range rng in doc.StoryRanges)
{
    bool hasFound = false;
    rng.Find.Replacement.ClearFormatting();
    rng.Find.Text = findWhat;
    rng.Find.Replacement.Text = replaceText;
    rng.Find.Forward = true;
    rng.Find.MatchCase = false;
    rng.Find.Wrap = Microsoft.Office.Interop.Word.WdFindWrap.wdFindStop;
    rng.Find.Format = true;
    hasFound = rng.Find.Execute(Replace:  
    Microsoft.Office.Interop.Word.WdReplace.wdReplaceAll);
}

When i execute function,it changes formatting of text,original text is underlined but it makes it plain. Any ideas? Thanks

My state is different; Original String -> ^tcookie, i want to make first letter after Tabs upper so i replace ^tc -> ^tC
When i do it, final text becomes ^tCookie. How can i fix it?