If I understand your question correctly, the comparison in KeyDown event works only with uppercase input.
If that is the case, the keycode is the same for upper and lower case characters. If you need to know is the letter in upper case you should investigae the shift property. However, if you want both upper and lower case characters to be accepted then make the comparison in upper case.
Something like
string keycode = e.KeyCode.ToString();
if (keycode == lettersInText[i].ToString().ToUpper())
{
...
To make it simpler you can store the whole initial text using upper case in the Load event if you like.