Hmm...learning as I go. We can combine the two ranges A-Z and a-z in
one pass:

Sub TTF2()
Dim oRng As Word.Range
Set oRng = ActiveDocument.Content
With oRng.Find
.MatchWildcards = True
.Text = "[A-Za-z]"
.Replacement.Text = "^&"
.Replacement.Font.Name = "FairScrollDisplay"
.Execute Replace:=wdReplaceAll
End With
End Sub