Public Function ReplaceNextChar(ByVal strPat As String, ByVal strSymb As String, ByVal strNew As String, ByVal firstOnly As Boolean) As String
    If strPat = "" Then Exit Function
    If strSymb = "" Then Exit Function
    ReplaceNextChar = ""
    Dim i As Integer, oldStrLen As Integer, holdStr As String, StrLoc As Integer
    oldStrLen = Len(strSymb)
    StrLoc = InStr(strPat, strSymb) +  1 
    While StrLoc >  1 
        holdStr = holdStr & Left(strPat, StrLoc -  1 ) & strNew
        strPat = Mid(strPat, StrLoc + oldStrLen)
        StrLoc = InStr(strPat, strSymb) +  1 
        If firstOnly Then ReplaceNextChar = holdStr & strPat: Exit Function
    Wend
    If Len(holdStr & strPat) > oldStrLen Then
        ReplaceNextChar = Left(holdStr & strPat, Len(holdStr & strPat) -  1 )
    Else
        ReplaceNextChar = holdStr & strPat
    End If
End Function

Sub Test ()
MsgBox ReplaceNextChar("asdfasdfasdf","s","#",False)
End Sub