I don't see any reference to textbox a named "NBL-02" in your code which, it seems to me, could be reduced to:
Code:
With ActiveDocument.Shapes("NBL-02").TextFrame.TextRange.Find
.Text = old_text
.Replacement.Text = "400"
.Forward = True
.Wrap = wdFindContinue
.Format = False
.MatchCase = False
.MatchWholeWord = False
.MatchWildcards = False
.MatchSoundsLike = False
.MatchAllWordForms = False
.Execute Replace:=wdReplaceAll
End With