You are overthinking this. Try the following
Code:
Sub footnoteCharacters()
'Graham Mayor - http://www.gmayor.com - Last updated - 07 Jun 2017
Dim oFNote As Footnote
Dim oRng As Range
Dim i As Integer, j As Integer
For i = ActiveDocument.Footnotes.Count To 1 Step -1
Set oFNote = ActiveDocument.Footnotes(i)
Set oRng = oFNote.Reference.Characters.Last
oRng.Text = " £$%" & oFNote.Range.Text & "%$£"
DoEvents
Next i
lbl_Exit:
Set oFNote = Nothing
Set oRng = Nothing
Exit Sub
End Sub