Option Explicit

Sub RandomLetters()
Dim oRng As Range
Dim oFind As Range
Dim i As Long
Dim strChar As String
Start:
    strChar = InputBox("Replace which character?", , "a")
    If Len(strChar) > 1 Or Not IsLetter(asc(strChar)) Then
        MsgBox "Enter a single character from the ranges A-Z or a-z only", vbCritical
        GoTo Start
    End If
    Set oFind = Selection.Range
    Set oRng = Selection.Range
    For i = 1 To oRng.Characters.Count
        With oRng.Find
            Do While .Execute(FindText:=strChar, MatchCase:=True)
                If oRng.InRange(oFind) Then
                    oRng.Text = RandomLetter(strChar)
                    oRng.Font.ColorIndex = wdRed
                End If
                oRng.Collapse wdCollapseEnd
            Loop
        End With
    Next i
End Sub

Private Function IsLetter(ByVal i As Integer) As Boolean
Select Case i
        Case 65 To 90, 97 To 122
            IsLetter = True
        Case Else
            IsLetter = False
    End Select
lbl_Exit:
    Exit Function
End Function

Private Function RandomLetter(strChar As String) As String
    Select Case asc(strChar)
        Case 65 To 90
            RandomLetter = Chr(65 + Rnd() * 10000000 Mod 26)
        Case 97 To 122
            RandomLetter = Chr(97 + Rnd() * 10000000 Mod 26)
        Case Else
            RandomLetter = strChar
    End Select
lbl_Exit:
    Exit Function
End Function