Tohle už bude fungovat.

var fromChars = new[] { 'a', 'b', 'c' };
var toChars = new[] { 'n', 'o', 'p' };
var result = new char[txt.Length];
for (var i = 0; i < txt.Length; i++)
{
    var idx = Array.IndexOf(fromChars, txt[i]);
    result[i] = idx > -1 ? toChars[idx] : txt[i];
}
Clipboard.SetText(new string(result));