There are 11 story ranges in a document. You need to loop through them and set the search range appropriately. This is a lot more complicated than the original.
The following should work.
Sub ConvertMicronSymbolInNormalFontToGreekLetterMuInSymbolFont()
'Graham Mayor - https://www.gmayor.com - Last updated - 21 Mar 2023
Dim rngStory As Word.Range
Dim oShp As Shape
Dim sCollect As String
Dim iJ As Double, iK As Double, iM As Double
iJ = 0: iK = 0: iM = 0
For Each rngStory In ActiveDocument.StoryRanges
Select Case rngStory.StoryType
Case 1 To 11
Do
sCollect = SrcAndRplInStory(rngStory)
iJ = iJ + Val(Split(sCollect, "|")(0))
iK = iK + Val(Split(sCollect, "|")(1))
iM = iM + Val(Split(sCollect, "|")(2))
On Error Resume Next
DoEvents
On Error GoTo 0
Select Case rngStory.StoryType
Case 6, 7, 8, 9, 10, 11
If rngStory.ShapeRange.Count > 0 Then
For Each oShp In rngStory.ShapeRange
If oShp.TextFrame.HasText Then
sCollect = SrcAndRplInStory(oShp.TextFrame.TextRange)
iJ = iJ + Val(Split(sCollect, "|")(0))
iK = iK + Val(Split(sCollect, "|")(1))
iM = iM + Val(Split(sCollect, "|")(2))
End If
DoEvents
Next oShp
End If
Case Else
'Do Nothing
End Select
On Error GoTo 0
'Get next linked story (if any)
Set rngStory = rngStory.NextStoryRange
Loop Until rngStory Is Nothing
Case Else
End Select
DoEvents
Next rngStory
lbl_Exit:
If iJ = 0 Then MsgBox "No matches found.", vbInformation
If iK > 0 Or iM > 0 Then MsgBox iK & " matches converted." & vbCr & _
iM & " matches skipped.", vbInformation
Set rngStory = Nothing
Set oShp = Nothing
Exit Sub
err_Handler:
Resume lbl_Exit
End Sub
Private Function SrcAndRplInStory(oRng As Range) As String
Dim vFindText As Variant
Dim vReplaceText As Variant
Dim i As Integer, j As Integer, k As Integer, m As Integer
Dim lAsk As Long
vFindText = Array(Chr(181))
vReplaceText = Array(-3987)
j = 0: k = 0: m = 0
For i = 0 To UBound(vFindText)
With oRng.Find
.Text = vFindText(i)
Do While .Execute
j = j + 1
oRng.Select
lAsk = MsgBox("Convert this character?", vbYesNoCancel)
If lAsk = 2 Then GoTo lbl_Exit
If lAsk = 6 Then
k = k + 1
oRng.InsertSymbol Font:="Symbol", _
CharacterNumber:=vReplaceText(i), _
Unicode:=True
Else
m = m + 1
End If
oRng.Collapse 0
Loop
End With
Next i
lbl_Exit:
SrcAndRplInStory = j & "|" & k & "|" & m
Exit Function
End Function