Selection.Find.ClearFormatting
Selection.Find.Replacement.ClearFormatting
    With Selection.Find
        .Text = "1"
        .Replacement.Text = "3"
        .Forward = True
        .Wrap = wdFindContinue
        .Format = False
        .MatchCase = False
        .MatchWholeWord = False
        .MatchWildcards = False
        .MatchSoundsLike = False
        .MatchAllWordForms = False
End With
Selection.Find.Execute Replace:=wdReplaceAll

...
var
   ...
   wordApp, wordAppSelection, wordAppFind: variant;
   ...
begin
...
   wordApp := CreateOleObject('Word.Application');
   wordApp.Documents.Open(fileName);
   wordApp.Visible := true;

   wordAppSelection := wordApp.Selection;
   wordAppFind := wordAppSelection.Find;
   wordAppFind.ClearFormatting;
   wordAppFind.Replacement.ClearFormatting;
   wordAppFind.Text := '1';
   wordAppFind.Replacement.Text := '3';
    
   // в этом месте вопрос
end

wordAppFind.Execute(Replace:=wdReplaceAll);